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Deep multiredshift limits on Epoch of Reionization 21 cm power spectra from four seasons of Murchison Widefield Array observations
Abstract
We compute the spherically averaged power spectrum from four seasons of data obtained for the Epoch of Reionization (EoR) project observed with the Murchison Widefield Array (MWA). We measure the EoR power spectrum over k = 0.07–3.0 h Mpc−1 at redshifts z = 6.5–8.7. The largest aggregation of 110 h on EoR0 high band (3340 observations), yields a lowest measurement of (43 mK)2 = 1.8 × 103 mK2 at k = 0.14 h Mpc−1 and z = 6.5 (2σ thermal noise plus sample variance). Using the Real-Time System to calibrate and the CHIPS pipeline to estimate power spectra, we select the best observations from the central five pointings within the 2013–2016 observing seasons, observing three independent fields and in two frequency bands. This yields 13 591 2-min snapshots (453 h), based on a quality assurance metric that measures ionospheric activity. We perform another cut to remove poorly calibrated data, based on power in the foreground-dominated and EoR-dominated regions of the two-dimensional power spectrum, reducing the set to 12 569 observations (419 h). These data are processed in groups of 20 observations, to retain the capacity to identify poor data, and used to analyse the evolution and structure of the data over field, frequency, and data quality. We subsequently choose the cleanest 8935 observations (298 h of data) to form integrated power spectra over the different fields, pointings, and redshift ranges.
... This was achieved using the ionospheric metric developed by Jordan et al. (2017), which incorporates the median source offset and source offset anisotropy derived from the measured versus expected source positions of 1 000 point sources in the field of view. Observations yielding an ionospheric metric greater than the cut-off threshold estimated in Trott et al. (2020) were excluded, resulting in 4 943 observations (equivalent to 165 h), as depicted by the green area in Fig. 3. The ionospheric distributions are illustrated in Fig. 7, with mean values ranging from 3.8 to 4.5 arcmin across pointings. ...
... The filtering process discussed in Section 6 yielded a set of 1 734 observations (58 h) from six pointings. Before delving into constructing the power spectrum from the observations, we compared our pipeline with the traditional MWA Real Time System pipeline (RTS, Mitchell et al. 2007) used in Trott et al. (2020). ...
... We overlaid upper limits from Trott et al. (2020) and Rahimi et al. (2021). However, these upper limits cannot be directly compared with our measurements due to differences in calculation conditions. ...
Observations of the 21 cm signal face significant challenges due to bright astrophysical foregrounds that are several orders of magnitude higher than the brightness of the hydrogen line, along with various systematics. Successful 21 cm experiments require accurate calibration and foreground mitigation. Errors introduced during the calibration process such as systematics can disrupt the intrinsic frequency smoothness of the foregrounds, leading to power leakage into the Epoch of Reionisation window. Therefore, it is essential to develop strategies to effectively address these challenges. In this work, we adopt a stringent approach to identify and address suspected systematics, including malfunctioning antennas, frequency channels corrupted by radio frequency interference, and other dominant effects. We implement a statistical framework that utilises various data products from the data processing pipeline to derive specific criteria and filters. These criteria and filters are applied at intermediate stages to mitigate systematic propagation from the early stages of data processing. Our analysis focuses on observations from the Murchison Widefield Array Phase I configuration. Out of the observations processed by the pipeline, our approach selects 18%, totalling 58 h, that exhibit fewer systematic effects. The successful selection of observations with reduced systematic dominance enhances our confidence in achieving 21 cm measurements.
... The spatial signatures are quantified through statistical measures such as the power spectrum, which can probe the 21-cm signal strength as a function of cosmological length scales (k-modes). Alongside, the three-dimensional topology of the EoR can be studied via a two-dimensional 21-cm power spectrum Trott et al. 2020;Mertens et al. 2020;The HERA Collaboration et al. 2021;Munshi et al. 2023), which shows the variation of the 21-cm power spectrum along the line of sight and transverse axis. ...
... Calibration accuracy is especially important for EoR observations using the MWA because of the presence of sharp periodic features in the bandpass produced by the polyphase filter bank used. The inability to accurately correct for these element-based bandpass structures significantly affects the power spectrum estimates (Beardsley et al. 2016; Barry et al. 2019;Trott et al. 2020;Patwa, Sethi, & Dwarakanath 2021;Yoshiura et al. 2021). ...
... MHz band. The fine polyphase filterbank shape results in poor bandpass characteristics at the coarse channel edges (Trott et al. 2020 not account for the diffuse sky emission. Then, we generated the foreground sky visibilities and converted them to MWA-style uvfits. ...
The radio interferometric closure phases can be a valuable tool for studying cosmological H I from the early Universe. Closure phases have the advantage of being immune to element-based gains and associated calibration errors. Thus, calibration and errors therein, which are often sources of systematics limiting standard visibility-based approaches, can be avoided altogether in closure phase analysis. In this work, we present the first results of the closure phase power spectrum of H I 21-cm fluctuations using the Murchison Widefield Array (MWA), with h of MWA phase II observations centred around redshift, , during the Epoch of Reionisation. On analysing three redundant classes of baselines – 14, 24, and 28 m equilateral triads, our estimates of the (95% confidence interval) 21-cm power spectra are at in the EoR1 field for the 14 m baseline triads, and at in the EoR0 field for the 24 m baseline triads. The ‘ pseudo ’ units denote that the length scale and brightness temperature should be interpreted as close approximations. Our best estimates are still 3-4 orders high compared to the fiducial 21-cm power spectrum; however, our approach provides promising estimates of the power spectra even with a small amount of data. These data-limited estimates can be further improved if more datasets are included into the analysis. The evidence for excess noise has a possible origin in baseline-dependent systematics in the MWA data that will require careful baseline-based strategies to mitigate, even in standard visibility-based approaches.
... For this purpose, multiple interferometric low-frequency radio telescopes have been designed, such as PAPER 1 , MWA 2 , HERA 3 , LOFAR 4 , and the upcoming SKA 5 . While the signal is yet to be detected, upper limits of the 21-cm power spectrum have been obtained and are becoming increasingly tighter (e.g., Mertens et al. 2020;Trott et al. 2020;HERA Collaboration et al. 2023;Acharya et al. 2024c). These have allowed us to rule out some extreme astrophysical models through comparisons with simulations (e.g., Ghara et al. 2020;Mondal et al. 2020;Greig et al. 2021a,b;Abdurashidova et al. 2022). ...
... These similarities in terms of 21-cm observables have a profound impact on inference modeling, as models with very different cosmological and astrophysical parameters may still produce 21-cm signal power spectra in agreement with current upper limits (for example from Mertens et al. 2020;Trott et al. 2020;HERA Collaboration et al. 2023;Acharya et al. 2024c), or possibly also with an eventual measurement. This is because, while the end of reionization may differ among models, at > 7 the extent and speed of reionization are still largely similar. ...
... UVLFs and 21-cm signal power spectrum). In particular, we note that despite significantly different galactic processes and reionization histories, the 21-cm power spectra are very similar in power across -bins and in agreement with current observational upper limits (Acharya et al. 2024c;Mertens et al. 2020;Trott et al. 2020;HERA Collaboration et al. 2023). Due to this, when doing inference modeling, it is essential to consider all cosmological and astrophysical parameters as free parameters, with other observational constraints serving as priors. ...
A detection of the 21-cm signal power spectrum from the Epoch of Reionization is imminent, thanks to consistent advancements from telescopes such as LOFAR, MWA, and HERA, along with the development of SKA. In light of this progress, it is crucial to expand the parameter space of simulations used to infer astrophysical properties from this signal. In this work, we explore the role of cosmological parameters such as the Hubble constant and the matter clustering amplitude , whose values as provided by measurements at different redshifts are in tension. We run N-body simulations using GADGET-4, and post-process them with the reionization simulation code POLAR, that uses L-GALAXIES to include galaxy formation and evolution properties and GRIZZLY to execute 1-D radiative transfer of ionizing photons in the intergalactic medium (IGM). We compare our results with the latest JWST observations and explore which astrophysical properties for different cosmologies are necessary to match the observed UV luminosity functions at redshifts and 9. Additionally, we explore the impact of these parameters on the observed 21-cm signal power spectrum, focusing on the redshifts within the range of LOFAR 21-cm signal observations (). Despite differences in cosmological and astrophysical parameters, the 21-cm power spectrum at these redshifts agrees with presently observed upper limits. This suggests the need for broader physical parameter spaces for inference modeling to account for all models that agree with observations. However, we also propose stronger constraining power by using a combination of galactic and IGM observables.
... The EDGES collaboration reported the detection of a feature in the sky-averaged spectrum (Bowman et al. 2018), potentially consistent with expectations for the global 21-cm signal, but strong enough to drive a considerable flurry of activity in the modeling community. Meanwhile, upper limits from power spectrum measurements have continued to improve in the last few years (Mertens et al. 2020;Trott et al. 2020;Abdurashidova et al. 2022a), and have recently breached the parameter space of "normal" models, i.e., those that do not invoke exotic mechanisms in order to amplify fluctuations beyond the theoretical maximum set by a ΛCDM cosmology. Arrays operating at higher frequencies, and so targeting neutral hydrogen in the post-reionization Universe, have reported auto-correlation and cross-correlation detections (Chang et al. 2010;Amiri et al. 2023;Paul et al. 2023), demonstrating both a matu-rity in the calibration and analyses of low-frequency radio interferometers (see review by Liu & Shaw 2020), and detection proof-of-principle, albeit in a frequency regime where foregrounds are weaker than those relevant to EoR studies. ...
... Colored contours along the right hand side represent constraints on the unresolved fraction of the cosmic X-ray background (in the soft band from Chandra; Lehmer et al. 2012), which disfavor very large values of /SFR. Indicated along the left is a region of parameter space in which models produce effectively no heating of the ∼ 8 IGM, a scenario which is now disfavoured by 21-cm power spectrum experiments (Abdurashidova et al. 2022a;Trott et al. 2020). Note that the X-ray and 21-cm contours are not confidence intervals; instead, they enclose regions in which all models violate the given constraint (dense cross-hatching) and regions where only some models remain consistent with these constraints (sparser cross-hatching). ...
... Clearly, stronger monopole absorption signals result in stronger dipole signals (lower right panel), which generally correspond to late features ≳ 110 MHz (left column). These are precisely the kinds of scenarios that 21-cm power spectrum limits are beginning to rule out (Abdurashidova et al. 2022b;HERA Collaboration et al. 2023;Trott et al. 2020). Models disfavoured by 21-cm and X-ray background constraints are not shown here, as they 'pile up' at the boundaries of the prior volume. ...
The 21-cm background is a promising probe of early star formation and black hole activity. While a slew of experiments on the ground seek to detect the 21-cm monopole and spatial fluctuations on large arcminute scales, little work has been done on the prospects for detecting the 21-cm dipole signal or its utility as a probe of early galaxies. Though an intrinsically weak signal relative to the monopole, its direction is known well from the cosmic microwave background and wide-field surveys, plus as a relative measurement the dipole could help relax instrumental requirements. In order to understand the constraining power of the dipole, in this work we perform parameter inference on mock datasets that include the dipole, monopole, or both signals. We find that while the monopole does provide the best constraints for a given integration time, constraints from a dipole measurement are competitive, and can in principle constrain the cosmic star formation rate density and efficiency of X-ray photon production in early galaxies to better than a factor of . This result holds for most of the available prior volume, which is set by constraints on galaxy luminosity functions, the reionization history, and upper limits from 21-cm power spectrum experiments. We also find that predictions for the monopole from a dipole measurement are robust to different choices of signal model. As a result, the 21-cm dipole signal is a valuable target for future observations and offers a robust cross-check on monopole measurements.
... This was achieved using the ionospheric metric developed by Jordan et al. 2017, which incorporates the median source offset and source offset anisotropy derived from the measured versus expected source positions of 1000 point sources in the field of view. Observations yielding an ionospheric metric greater than the cut-off threshold estimated in Cathryn M. Trott et al. 2020 were excluded, resulting in 4943 observations (equivalent to 165 hours), as depicted by the green area in Figure 2. The ionospheric distributions are illustrated in Figure 7, with mean values ranging from 3.8 to 4.5 arcminutes across pointings. This metric serves as a proxy for ionospheric activity in an observation, with lower values indicating less ionospheric activity. ...
... mK) 2 and ∆ 2 = (74.6 mK) 2 at k = 0.19 h Mpc -1 respectively. We overlaid upper limits from Cathryn M. Trott et al. (2020) and Rahimi et al. (2021). However, these upper limits cannot be directly compared with our measurements due to differences in calculation conditions. ...
... Rahimi et al. (2021) utilised only phase I data and focused on a different observing field. On the other hand, Cathryn M. Trott et al. (2020) used observations from the same field but included both phase I and phase II data, employing a different sky model. Incorporating the sky model from Cathryn M. Trott et al. (2020) yielded marginal differences. ...
Observations of the 21 cm signal face significant challenges due to bright astrophysical foregrounds that are several orders of magnitude higher than the brightness of the hydrogen line, along with various systematics. Successful 21 cm experiments require accurate calibration and foreground mitigation. Errors introduced during the calibration process such as systematics, can disrupt the intrinsic frequency smoothness of the foregrounds, leading to power leakage into the Epoch of Reionisation (EoR) window. Therefore, it is essential to develop strategies to effectively address these challenges. In this work, we adopt a stringent approach to identify and address suspected systematics, including malfunctioning antennas, frequency channels corrupted by radio frequency interference (RFI), and other dominant effects. We implement a statistical framework that utilises various data products from the data processing pipeline to derive specific criteria and filters. These criteria and filters are applied at intermediate stages to mitigate systematic propagation from the early stages of data processing. Our analysis focuses on observations from the Murchison Widefield Array (MWA) Phase I configuration. Out of the observations processed by the pipeline, our approach selects 18%, totalling 58 hours, that exhibit fewer systematic effects. The successful selection of observations with reduced systematic dominance enhances our confidence in achieving 21 cm measurements.
... The spatial signatures are quantified through statistical measures such as the power spectrum, which can probe the 21-cm signal strength as a function of cosmological length scales (k-modes). Alongside, the threedimensional topology of the EoR can be studied via a two-dimensional 21-cm power spectrum Trott et al. 2020;Mertens et al. 2020;The HERA Collaboration et al. 2021;Munshi et al. 2023), which shows the variation of the 21-cm power spectrum along the line of sight and transverse axis. ...
... Calibration accuracy is especially important for EoR observations using the MWA because of the presence of sharp periodic features in the bandpass produced by the polyphase filter bank used. The inability to accurately correct for these element-based bandpass structures significantly affects the power spectrum estimates (Beardsley et al. 2016; Barry, Wilensky, and al. 2019;Trott et al. 2020;Patwa, Sethi, and Dwarakanath 2021;Yoshiura et al. 2021). ...
... MHz band. The fine polyphase filterbank shape results in poor bandpass characteristics at the coarse channel edges (Trott et al. 2020). ...
The radio interferometric closure phases can be a valuable tool for studying cosmological {H\scriptsize{I}}~from the early Universe. Closure phases have the advantage of being immune to element-based gains and associated calibration errors. Thus, calibration and errors therein, which are often sources of systematics limiting standard visibility-based approaches, can be avoided altogether in closure phase analysis. In this work, we present the first results of the closure phase power spectrum of {H\scriptsize{I}}~21-cm fluctuations using the Murchison Widefield Array (MWA), with hours of MWA-phase II observations centered around redshift, , during the Epoch of Reionisation. On analysing three redundant classes of baselines -- 14~m, 24~m, and 28~m equilateral triads, our estimates of the ( confidence interval) 21-cm power spectra are at in the EoR1 field for the 14~m baseline triads, and at in the EoR0 field for the 24~m baseline triads. The ``pseudo'' units denote that the length scale and brightness temperature should be interpreted as close approximations. Our best estimates are still 3-4 orders high compared to the fiducial 21-cm power spectrum; however, our approach provides promising estimates of the power spectra even with a small amount of data. These data-limited estimates can be further improved if more datasets are included into the analysis. The evidence for excess noise has a possible origin in baseline-dependent systematics in the MWA data that will require careful baseline-based strategies to mitigate, even in standard visibility-based approaches.
... Conversely, interferometers focusing on statistical fluctuations have yielded significant upper limits on the EoR power spectrum (PS) amplitude. The most sensitive operational interferometers, such as the GMRT, MWA, LOFAR, and HERA, have all established upper limits on the power spectrum amplitude of the signal [22][23][24][25][26][27], but there has been no confirmed detection of the cosmological Hi 21cm signal. ...
... A key scientific objective of both ongoing and upcoming radio interferometric arrays is to detect and characterize fluctuations in the brightness temperature of the redshifted 21cm sig- nal from the EoR. Various statistical tools are available for these interferometric experiments to calculate these fluctuations in terms of the power spectrum (PS) using both the visibility data [25] and the image plane [26]. In this study, we have calculated the theoretical PS from the image plane and the observed PS from simulated visibility data for different arrays such as SKA Low and HERA, as shown in Fig. 2. ...
The Epoch of Reionization (EoR) and Cosmic Dawn (CD) are pivotal stages during the first billion years of the universe, exerting a significant influence on the development of cosmic structure. The detection of the redshifted 21-cm signal from these epochs is challenging due to the dominance of significantly stronger astrophysical foregrounds and the presence systematics. This work used the 21cm E2E pipeline, followed by simulation methodology described \cite{2022Mazumder} to conduct synthetic observations of a simulated sky model that includes both the redshifted 21-cm signal and foregrounds. A framework was constructed using Artificial Neural Networks (ANN) and Bayesian techniques to directly deduce astrophysical parameters from the measured power spectrum. This approach eliminates the need for explicit telescope effects correction in interferometric arrays such as SKA-Low and HERA. The present work investigates the impact of calibration and position errors on retrieving the redshifted 21-cm power spectrum for the above arrays. We assessed the effects of these inaccuracies on the deduced astrophysical parameters and established acceptable tolerance levels. Based on our results, the calibration error tolerance for ideal signal detection is 0.001 %. However, if the position errors exceed 0.048 arcseconds, the remaining foregrounds would obscure the target signal.
... The field of observational 21-cm cosmology is rapidly evolving with ongoing experiments [e.g. 33,34,[46][47][48][49] progressively improving upper limits, and the under-construction SKA scheduled for commissioning in 2028. Here we consider prospective constraints on the Pop III IMF using the sensitivities of the operational REACH global signal experiment and SKA as examples. ...
... 33,46] or power spectrum detections [e.g. 34,[47][48][49], the prospect for 21-cm cosmology to measure the masses of the first stars in the coming years is very promising. ...
The formation of the first stars and the subsequent population of X-ray binaries represents a fundamental transition in the state of the Universe as it evolves from near homogeneity to being abundant in collapsed structures such as galaxies. Due to a lack of direct observations, the properties of these stars remain highly uncertain. By considering the impact of the first stars and their remnant X-ray binaries on the cosmological 21-cm signal, we demonstrate that upcoming observations have the potential to significantly improve our understanding of these objects. We find a 25 mK sensitivity measurement of the 21-cm global signal by a wide-beam radiometer, such as REACH, or 3,000 hours of foreground avoidance observations of the 21-cm power spectrum by SKA-Low, could provide three-sigma constraints on the mass distribution of the first stars. Such measurements will fill a critical gap in our understanding of the early Universe and aid in interpreting high-redshift galaxy observations.
... Crucially, these are synchrotron and free-free emitters, with continuum spectra, allowing a spectral distinction to be made between them and the spectrally structured 21-cm line emission. This fundamental difference forms the basis for discriminating foreground contaminating power from the signal of interest for all experiments attempting this measurement (Koopmans et al. 2015;HERA Collaboration et al. 2023;Beardsley et al. 2016;Trott et al. 2020; Barry et al. 2019;Patil et al. 2017;Mertens et al. 2020). The brightest and closest of these foreground sources can be resolved and measured individually, and these form the basis for a sky-based approach to calibrating low-frequency radio data. ...
... We only simulate a single pointing (zenith) to reduce computational load, particularly with the diffuse and 21-cm emission. In Trott et al. (2020), the behaviour of individual versus combined pointings in the PS did not show significant differences, justifying use of a single pointing for this work. We simulate all data at a 2 s, 40 kHz to match the resolution of real data, ensuring any effects of averaging to 8 s, 80 kHz when applying calibration solutions or creating power spectra are captured. ...
Detection of the weak cosmological signal from high-redshift hydrogen demands careful data analysis and an understanding of the full instrument signal chain. Here, we use the WODEN simulation pipeline to produce realistic data from the Murchison Widefield Array (MWA) Epoch of Reionisation experiment and test the effects of different instrumental systematics through the AusEoRPipe analysis pipeline. The simulations include a realistic full sky model, direction-independent calibration, and both random and systematic instrumental effects. Results are compared to matched real observations. We find that, (i) with a sky-based calibration and power spectrum approach we have need to subtract more than 90% of all unresolved point source flux (10 mJy apparent) to recover 21-cm signal in the absence of instrumental effects; (ii) when including diffuse emission in simulations, some k -modes cannot be accessed, leading to a need for some diffuse emission removal; (iii) the single greatest cause of leakage is an incomplete sky model; and (iv) other sources of errors, such as cable reflections, flagged channels, and gain errors, impart comparable systematic power to one another and less power than the incomplete sky model.
... During the Cosmic Dawn and the Epoch of Reionization (EoR), at redshifts z ∼ 5 − 30, the 21-cm line will provide us with tomographic maps of those redshifts and thus potentially new standard rulers. This signal, emitted by the spin-flip transition of neutral hydrogen, is the target of radio telescopes such as the Hydrogen Epoch of Reionization Array (HERA; HERA Collaboration et al. [17]), the LOw Frequency ARray (LO-FAR; Mertens et al. [18]), the Murchison Widefield Array (MWA; Trott et al. [19]), the Precision Array for Probing the Epoch of Re-ionization (PAPER; Parsons et al. [20], or the Long Wavelength Array (LWA; Dilullo et al. [21]). Muñoz [22] proposed that velocity-induced acoustic oscillations (VAOs) provide a new (calibrated) standard ruler that can constrain cosmology during Cosmic Dawn (15 ≲ z ≲ 20; see also [23,24]). ...
... Current observations are setting limits on the 21-cm power spectrum, and are inching towards a detection of this observable [17][18][19]. However, a next-generation instrument like the SKA is expected to obtain 21-cm lightcones (i.e., 3D maps), from which to extract ionized bubbles. ...
In this paper, we propose an Alcock-Paczy\'nski (AP) test to constrain cosmology using HII bubbles during the Epoch of Reionization. Similarly to cosmic voids, a stack of HII bubbles is spherically symmetric because ionizing fronts propagate isotropically on average (even if individual bubbles may not be spherical), making them standard spheres to be used in an AP test. Upcoming 21-cm observations, from the Square Kilometer Array (SKA) for instance, will contain tomographic information about HII regions during reionization. However, extracting the bubbles from this signal is made difficult because of instrumental noise and foreground systematics. Here, we use a neural network to reconstruct neutral-fraction boxes from the noisy 21-cm signal, from which we extract bubbles using a watershed algorithm. We then run the purely geometrical AP test on these stacks, showing that a SKA-like experiment will be able to constrain the product of the angular-diameter distance and Hubble parameter H at reionization redshifts with precision, robustly to astrophysical and cosmological uncertainties within the models tested here. This AP test, whether performed on 21-cm observations or other large surveys of ionized bubbles, will allow us to fill the knowledge gap about the expansion rate of our Universe at reionization redshifts.
... Disregarding the spectral axis for now, a rough requirement would hold that the coupling must contribute less than the expected ratio of the foreground power spectrum to the 21 cm signal. Recent limits reported by Trott et al. (2020) report foreground levels at 1 15 m 2 and forecasts call for power spectrum levels of 100 m 2 . This gives a ratio of 1 14 in power spectrum, 1 7 in visibility, 1 3.5 in voltage, or -35 dB. for three NS polarized tiles with a 14 m baseline and the sum of all baselines. ...
... We approximate that mutual coupling must be less than -35 dB in order to avoid foreground leakage into the EoR window (Trott et al. 2020). The current models show that it is only above 240 MHz that mutual coupling exceeds that limit, which is above the frequency range for the EoR. ...
Measurement of the power spectrum of high redshift 21 cm emission from neutral hydrogen probes the formation of the first luminous objects and the ionization of intergalactic medium by the first stars. However, the 21 cm signal at these redshifts is orders of magnitude fainter than astrophysical foregrounds, making it challenging to measure. Power spectrum techniques may be able to avoid these foregrounds by extracting foreground-free Fourier modes, but this is exacerbated by instrumental systematics that can add spectral structure to the data, leaking foreground power to the foreground-free Fourier modes. It is therefore imperative that any instrumental systematic effects are properly understood and mitigated. One such systematic occurs when neighboring antennas have undesired coupling. A systematic in Phase II data from the MWA was identified which manifests as excess correlation in the visibilities. One possible explanation for such an effect is mutual coupling between antennas. We have built a numerical electromagnetic software simulation of the antenna beam using FEKO to estimate the amplitude of this effect for multiple antennas in the MWA. We also calculate coupling predicted by the re-radiation model which is found to be somewhat lower than the coupling coefficients produced by the simulation. We find that with many approximations both types of mutual coupling predictions are somewhat lower than the minimum necessary to detect the brightest 21 cm models. However more work is necessary to better validate the required level of coupling and to verify that approximations did not under estimate the level of coupling.
... The period following the recombination era ( ∼ 1100) represents one of the last unexplored frontiers in modern astronomy and cosmology. The epochs starting after the cosmic Dark Ages ( > 30), those by Trott et al. (2020), who reported a 2-upper limit of Δ 2 21 < (43.9 mK) 2 at ≈ 6.5 and = 0.15 ℎ cMpc −1 with the MWA, using 298 h of carefully selected data, and the HERA team, who recently reported a 2-upper limit of Δ 2 21 < (30.76 mK) 2 at ≈ 7.9 (HERA Collaboration et al. 2023) and = 0.34 ℎ cMpc −1 using 94 nights of observations. The LOFAR-EoR Key Science Project has also made significant progress. ...
We present new upper limits on the 21-cm signal power spectrum from the Epoch of Reionization (EoR), at redshifts , based on reprocessed observations from the Low-Frequency Array (LOFAR). The analysis incorporates significant enhancements in calibration methods, sky model subtraction, radio-frequency interference (RFI) mitigation, and an improved signal separation technique using machine learning to develop a physically motivated covariance model for the 21-cm signal. These advancements have markedly reduced previously observed excess power due to residual systematics, bringing the measurements closer to the theoretical thermal noise limit across the entire k-space. Using comparable observational data, we achieve a 2 to 4-fold improvement over our previous LOFAR limits, with best upper limits of at , at and at at redshifts , and 8.3, respectively. These new multi-redshift upper limits provide new constraints that can be used to refine our understanding of the astrophysical processes during the EoR. Comprehensive validation tests, including signal injection, were performed to ensure the robustness of our methods. The remaining excess power is attributed to residual foreground emissions from distant sources, beam model inaccuracies, and low-level RFI. We discuss ongoing and future improvements to the data processing pipeline aimed at further reducing these residuals, thereby enhancing the sensitivity of LOFAR observations in the quest to detect the 21-cm signal from the EoR.
... Because the current generation of instruments does not have the sensitivity to make direct images of the 21-cm signal, they mainly focus on estimating the spatial fluctuations of the 21-cm signal by measuring its power spectrum. Increasingly more stringent upper limits have been set on the 21-cm power spectrum from the EoR by radio interferometers such as LOFAR ) and MWA (Trott et al. 2020;Kolopanis et al. 2023) with the highest frequency band, GMRT (Paciga et al. 2013), PAPER 3 (Parsons et al. 2010;Kolopanis et al. 2019), andHERA 4 (DeBoer et al. 2017;Abdurashidova et al. 2022), and from the CD by instruments such as LOFAR (Gehlot et al. 2019) and MWA (Yoshiura et al. 2021) with the lowest frequency band, OVRO-LWA 5 (Greenhill & Bernardi 2012;Eastwood et al. 2019), NenuFAR 6 (Zarka et al. 2012;Munshi et al. 2024), and AARTFAAC 7 (Gehlot et al. 2020). The next generation of instruments, such as HERA and SKA-Low 8 (Dewdney et al. 2009;Koopmans et al. 2015) are expected to have the sensitivity to detect the cosmological signal via power spectra, and SKA-Low has the potential to directly make tomographic images (e.g. ...
Studying the redshifted 21-cm signal from the the neutral hydrogen during the Epoch of Reionization and Cosmic Dawn is fundamental for understanding the physics of the early universe. One of the challenges that 21-cm experiments face is the contamination by bright foreground sources, such as Cygnus A, for which accurate spatial and spectral models are needed to minimise the residual contamination after their removal. In this work, we develop a new, high-resolution model of Cygnus A using Low Frequency Array (LOFAR) observations in the MHz range, improving upon previous models by incorporating physical spectral information through the forced-spectrum method during multi-frequency deconvolution. This approach addresses the limitations of earlier models by providing a more accurate representation of the complex structure and spectral behaviour of Cygnus A, including the spectral turnover in its brightest hotspots. The impact of this new model on the LOFAR 21-cm signal power spectrum is assessed by comparing it with both simulated and observed North Celestial Pole data sets. Significant improvements are observed in the cylindrical power spectrum along the Cygnus A direction, highlighting the importance of having spectrally accurate models of the brightest foreground sources. However, this improvement is washed out in the spherical power spectrum, where we measure differences of a few hundred mK at , but not statistically significant. The results suggest that other systematic effects must be mitigated before a substantial impact on 21-cm power spectrum can be achieved.
... Additionally, radio interferometers like PAPER [5], the MWA [6], LOFAR [7], HERA [8], and the upcoming Square Kilometre Array [9] will measure the frequency-dependent spatial variations of the 21-cm signal. Current power-spectrum limits are cutting into physically motivated parameter space [10][11][12][13], and upcoming observations are expected to reach deep enough to detect the reionization signal. ...
The 21-cm signal provides a novel avenue to measure the thermal state of the Universe during cosmic dawn and reionization (redshifts z ∼ 5 – 30 ), and thus to probe energy injection from decaying or annihilating dark matter (DM). These DM processes are inherently inhomogeneous: both decay and annihilation are density-dependent, and furthermore, the fraction of injected energy that is deposited at each point depends on the gas ionization and density, leading to further anisotropies in absorption and propagation. In this work, we develop a new framework for modeling the impact of spatially inhomogeneous energy injection and deposition during cosmic dawn, accounting for ionization and baryon density dependence, as well as the attenuation of propagating photons. We showcase how this first completely inhomogeneous treatment affects the predicted 21-cm power spectrum in the presence of exotic sources of energy injection, and forecast the constraints that upcoming HERA measurements of the 21-cm power spectrum will set on DM decays to photons and to electron/positron pairs. These projected constraints considerably surpass those derived from CMB and Lyman- α measurements, and for decays to electron/positron pairs they exceed all existing constraints in the sub-GeV mass range, reaching lifetimes of ∼ 10 28 s . Our analysis demonstrates the unprecedented sensitivity of 21-cm cosmology to exotic sources of energy injection during the cosmic dark ages. Our code, 21cm, includes all these effects and is publicly available in an accompanying release.
Published by the American Physical Society 2025
... There has also been significant progress in performing Hi IM with interferometers. While there have been upper limits placed on the Hi power spectrum at very high redshift (7 ≲ ≲ 15) using telescopes like uGMRT, MWA, HERA and LOFAR (Paciga et al. 2011;Mertens et al. 2020;Trott et al. 2020;Abdurashidova et al. 2022), no detections have yet been reported. At lower redshifts (0.78≲ ≲ 2.3), the Canadian Hydrogen Intensity Mapping Experiment (CHIME, Bandura et al. 2014) has detected large-scale structures with Hi stacking using Extended Baryon Oscillation Spectroscopic Survey (eBOSS, Dawson et al. 2016) data of galaxies between 0.78 ≲ ≲ 1.43 (Amiri et al. 2023) and Ly forest (CHIME Collaboration et al. 2023) at = 2.3. ...
We present the first results of the HI intensity mapping power spectrum analysis with the MeerKAT International GigaHertz Tiered Extragalactic Exploration (MIGHTEE) survey. We use data covering 4 square degrees in the COSMOS field using a frequency range 962.5 MHz to 1008.42 MHz, equivalent to HI emission in . The data consists of 15 pointings with a total of 94.2 hours on-source. We verify the suitability of the MIGHTEE data for HI intensity mapping by testing for residual systematics across frequency, baselines and pointings. We also vary the window used for HI signal measurements and find no significant improvement using stringent Fourier mode cuts. Averaging in the power spectrum domain, i.e. using incoherent averaging, we calculate the first upper limits from MIGHTEE on the HI power spectrum at scales . We obtain the best 1 upper limit of 28.6 mKMpc on 2 Mpc. Our results are consistent with the power spectrum detected with observations in the DEEP2 field with MeerKAT. The data we use here constitutes a small fraction of the MIGHTEE survey and demonstrates that combined analysis of the full MIGHTEE survey can potentially detect the HI power spectrum at in the range or quasi-linear scales.
... Modern interferometers like HERA, LOFAR, and the MWA have been designed to observe with many antennas simultaneously to maximize their sensitivity to the 21-cm signal from the EoR. These experiments have shown that detecting this signal is rife with difficulty (Abdurashidova et al., 2022;Mertens et al., 2020;Trott et al., 2020). This is primarily due to the coupling of bright contaminating sources between us and the cosmological signal, referred to as "foregrounds", with the spectral structure imparted by the instrument. ...
BayesEoR is a GPU-accelerated, MPI-compatible Python package for estimating the power spectrum of redshifted 21-cm emission from interferometric observations of the Epoch of Reionization (EoR). Utilizing a Bayesian framework, BayesEoR jointly fits for the 21-cm EoR power spectrum and a "foreground" model, referring to bright, contaminating emission between us and the cosmological signal, and forward models the instrument with which these signals are observed. To perform the sampling, we use MultiNest [arXiv:1402.0004], which calculates the Bayesian evidence as part of the analysis. Thus, BayesEoR can also be used as a tool for model selection [see e.g. arXiv:1701.03384].
... We only simulate a single pointing (zenith) to reduce computational load, particularly with the diffuse and 21-cm emission. In Trott et al. (2020), the behaviour of individual versus combined pointings in the PS did not show significant differences, justifying use of a single pointing for this work. We simulate all data at a 2s, 40kHz to match the resolution of real data, ensuring any effects of averaging to 8s, 80kHz when applying calibration solutions or creating power spectra are captured. ...
Detection of the weak cosmological signal from high-redshift hydrogen demands careful data analysis and an understanding of the full instrument signal chain. Here we use the WODEN simulation pipeline to produce realistic data from the Murchison Widefield Array Epoch of Reionisation experiment, and test the effects of different instrumental systematics through the AusEoRPipe analysis pipeline. The simulations include a realistic full sky model, direction-independent calibration, and both random and systematic instrumental effects. Results are compared to matched real observations. We find that, (i) with a sky-based calibration and power spectrum approach we have need to subtract more than 90% of all unresolved point source flux (10mJy apparent) to recover 21-cm signal in the absence of instrumental effects; (ii) when including diffuse emission in simulations, some k-modes cannot be accessed, leading to a need for some diffuse emission removal; (iii) the single greatest cause of leakage is an incomplete sky model; (iv) other sources of errors, such as cable reflections, flagged channels and gain errors, impart comparable systematic power to one another, and less power than the incomplete skymodel.
... In Paciga et al. (2013), the Giant Metrewave Radio Telescope 1 (GMRT, Ananthakrishnan 1995) Epoch of Reionization experiment reported ∆ 2 21 < (248) 2 mK 2 at k = 0.50 h cMpc −1 and z ≈ 8.6 in the 21-cm power spectrum. The latest 5.5-hours observation from the Murchison Widefield Array 2 (MWA, Barry et al. 2019;Li et al. 2019;Trott et al. 2020) reported ∆ 2 21 < 6.3 × (10) 2 mK 2 at k = 0.14 h cMpc −1 and z ≈ 15.2 in the 21-cm power spectrum in Yoshiura et al. (2021). Using a 4-hour observation from the Owen's Valley Radio Observatory -Long Wavelength Array 3 (OVRO-LWA, Eastwood et al. 2019), Garsden et al. (2021) reported an upper limit of ∆ 2 21 < 2 × (10) 12 mK 2 at k = 0.3 h cMpc −1 and z = 28 on the 21-cm power spectrum. ...
Neutral hydrogen serves as a crucial probe for the Cosmic Dawn and the Epoch of Reionization (EoR). Actual observations of the 21-cm signal often encounter challenges such as thermal noise and various systematic effects. To overcome these challenges, we simulate SKA-Low-depth images and process them with a deep learning method. We utilized foreground residuals acquired by LOFAR during actual North Celestial Pole field observations, thermal and excess variances calculated via Gaussian process regression, and 21-cm signals generated with 21cmFAST for signal extraction tests. Our approach to overcome these foreground, thermal noise, and excess variance components employs a 3D U-Net neural network architecture for image analysis. When considering thermal noise corresponding to 1400 hours of integration, U-Net provides reliable 2D power spectrum predictions, and robustness tests ensure that we get realistic EoR signals. Adding foreground residuals, however, causes inconsistencies below the horizon delay-line. Lastly, evaluating both thermal and excess variance with observations up to 3700 and 14000 hours ensures reliable power spectrum estimations within the EoR window and across nearly all scales, respectively.
... Given that other 21-cm line observables, such as 21-cm power spectrum from tomographic observations (e.g. Mertens et al. 2020;Trott et al. 2020; HERA Collaboration 2022) and sky-averaged 21-cm spectrum (e.g. Bowman et al. 2018;Singh et al. 2022), utilize Cosmic Microwave Background as the radio background source, the 21-cm forest signal is subject to different systematic uncertainties (Carilli et al. 2002;Furlanetto & Loeb 2002;Furlanetto et al. 2006; Pritchard & Loeb ★ E-mail: tomas.soltinsky@inaf.it ...
The 21-cm forest signal is a promising probe of the Epoch of Reionization complementary to other 21-cm line observables and Ly forest signal. Prospects of detecting it have significantly improved in the last decade thanks to the discovery of more than 30 radio-loud quasars at these redshifts, upgrades to telescope facilities, and the notion that neutral hydrogen islands persist down to . We forward-model the 21-cm forest signal using semi-numerical simulations and incorporate various instrumental features to explore the potential of detecting the 21-cm forest at z=6, both directly and statistically, with the currently available (uGMRT) and forthcoming (SKA1-low) observatories. We show that it is possible to detect the 1D power spectrum of the 21-cm forest spectrum, especially at large scales of with the of the uGMRT time and with the SKA1-low over if the intergalactic medium (IGM) is neutral and these neutral hydrogen regions have a spin temperature of . On the other hand, we infer that a null-detection of the signal with such observations of 10 radio-loud sources at can be translated into constraints on the thermal and ionization state of the IGM which are tighter than the currently available measurements. Moreover, a null-detection of the 1D 21-cm forest power spectrum with only of the uGMRT observations of 10 radio-loud sources can already be competitive with the Ly forest and 21-cm tomographic observations in disfavouring models of significantly neutral and cold IGM at z=6.
... Modern interferometers like HERA, LOFAR, and the MWA have been designed to observe with many antennas simultaneously to maximize their sensitivity to the 21-cm signal from the EoR. These experiments have shown that detecting this signal is rife with difficulty (Abdurashidova et al., 2022;Mertens et al., 2020;Trott et al., 2020). This is primarily due to the coupling of bright contaminating sources between us and the cosmological signal, referred to as "foregrounds", with the spectral structure imparted by the instrument. ...
... Examples of these experiments include the Donald C. Baker Precision Array for Probing the EoR (PAPER, Parsons et al. 2010), the LOw Frequency Arrray (LOFAR, van Haarlem et al. 2013), the New extension in Nançay upgrading LOFAR (NenuFAR, Zarka et al. 2012), the Murchison Widefield Array (MWA, Tingay et al. 2013;Wayth et al. 2018), the (upgraded) Giant Metrewave Radio Telescope (GMRT, Paciga et al. 2013;Gupta et al. 2017), the MeerKAT telescope (Santos et al. 2016), the Canadian Hydrogen Intensity Mapping Experiment (CHIME, CHIME Collaboration et al. 2022), the Hydrogen Epoch of Reionization Array (HERA, DeBoer et al. 2017;Berkhout et al. 2024), the Canadian Hydrogen Observatory and Radio-transient Detector (CHORD, Vanderlinde et al. 2019), the Owens Valley Long Wavelength Array (OVRO-LWA, Eastwood et al. 2019), the Hydrogen Intensity and Real-time Analysis eXperiment (HI-RAX, Crichton et al. 2022), and the Square Kilometre * CITA National Fellow † NASA Hubble Fellow Array (SKA, Santos et al. 2015;Koopmans et al. 2015). While none of these experiments have reported a direct detection of the 21cm power spectrum on its own beyond redshift z ∼ 1 so far (Paul et al. 2023), many efforts have been put in to derive sensitive upper limits on the 21cm power spectrum at various redshifts (Ghosh et al. 2011;Paciga et al. 2013;Parsons et al. 2014;Dillon et al. 2014Dillon et al. , 2015Ewall-Wice et al. 2016;Beardsley et al. 2016;Patil et al. 2017;Li et al. 2019; Barry et al. 2019;Eastwood et al. 2019;Gehlot et al. 2019;Kolopanis et al. 2019;Trott et al. 2020;Mertens et al. 2020;Chakraborty et al. 2021;Garsden et al. 2021;Yoshiura et al. 2021;Abdurashidova et al. 2022;HERA Collaboration et al. 2023;Wilensky et al. 2023;Munshi et al. 2024). ...
The precise characterization and mitigation of systematic effects is one of the biggest roadblocks impeding the detection of the fluctuations of cosmological 21cm signals. Missing data in radio cosmological experiments, often due to radio frequency interference (RFI), poses a particular challenge to power spectrum analysis as it could lead to the ringing of bright foreground modes in Fourier space, heavily contaminating the cosmological signals. Here we show that the problem of missing data becomes even more arduous in the presence of systematic effects. Using a realistic numerical simulation, we demonstrate that partially flagged data combined with systematic effects can introduce significant foreground ringing. We show that such an effect can be mitigated through inpainting the missing data. We present a rigorous statistical framework that incorporates the process of inpainting missing data into a quadratic estimator of the 21cm power spectrum. Under this framework, the uncertainties associated with our inpainting method and its impact on power spectrum statistics can be understood. These results are applied to the latest Phase II observations taken by the Hydrogen Epoch of Reionization Array, forming a crucial component in power spectrum analyses as we move toward detecting 21cm signals in the ever more noisy RFI environment.
... ion Array for Probing the Epoch of Reionization (PAPER; A. R. Parsons et al. 2010;C. Cheng et al. 2018; M. Kolopanis et al. 2019), the Owens Valley Long Wavelength Array (M. W. Eastwood et al. 2019;H. Garsden et al. 2021), the Murchison Widefield Array (MWA; S. J. Tingay et al. 2013;A. Ewall-Wice et al. 2016; N. Barry et al. 2019;W. Li et al. 2019;C. M. Trott et al. 2020;M. Rahimi et al. 2021;S. Yoshiura et al. 2021;C. M. Trott et al. 2022; M. J. Wilensky et al. 2023b), and the New Extension in Nançay Upgrading LOFAR (S. Munshi et al. 2024). These upper limits have begun to constrain theoretical models (R. Ghara et al. 2020;B. Greig et al. 2021;Z. Abdurashidova et al. 2022b;HERA Collaboration et al. 2023 ...
Intensity mapping—the large-scale mapping of selected spectral lines without resolving individual sources—is quickly emerging as an efficient way to conduct large cosmological surveys. Multiple surveys covering a variety of lines (such as the hydrogen 21 cm hyperfine line, carbon-monoxide rotational lines, and [C ii ] fine-structure lines, among others) are either observing or will soon be online, promising a panchromatic view of our Universe over a broad redshift range. With multiple lines potentially covering the same volume, cross-correlations have become an attractive prospect, both for probing the underlying astrophysics and for mitigating observational systematics. For example, cross-correlating 21 cm and [C ii ] intensity maps during reionization could reveal the characteristic scale of ionized bubbles around the first galaxies, while simultaneously providing a convenient way to reduce independent foreground contaminants between the two surveys. However, many of the desirable properties of cross-correlations in principle emerge only under ideal conditions, such as infinite ensemble averages. In this paper, we construct an end-to-end pipeline for analyzing intensity mapping cross-correlations, enabling instrumental effects, foreground residuals, and analysis choices to be propagated through Monte Carlo simulations to a set of rigorous error properties, including error covariances, window functions, and full probability distributions for power-spectrum estimates. We use this framework to critically examine the applicability of simplifying assumptions such as the independence and Gaussianity of power-spectrum errors. As worked examples, we forecast the sensitivity of near-term and futuristic 21 cm × [C ii ] cross-correlation measurements, providing recommendations for survey design.
... Numerous low-frequency radio experiments ( ≲ 225 MHz, corresponding to ≳ 5.3) have pursued measurements of the sky-averaged (i.e., global; Shaver et al. 1999) 21 cm signal (Bowman et al. 2018;Singh et al. 2018Singh et al. , 2022, as well as its Corresponding author: J. Dorigo Jones johnny.dorigojones@colorado.edu power spectrum (Paciga et al. 2011;Mertens et al. 2020;Trott et al. 2020;Garsden et al. 2021;HERA Collaboration et al. 2023), although systematic effects, mainly from the galactic foreground in combination with beam chromaticity and radio frequency interference (RFI), have so far prevented a clear detection of the global signal (e.g., Hills et al. 2018;Bradley et al. 2019;Sims & Pober 2020;Tauscher et al. 2020). Efforts are also underway to measure the Dark Ages 21 cm signal from the Moon (e.g., LuSEE-Night, Bale et al. 2023). ...
Neural network (NN) emulators of the global 21 cm signal need emulation error much less than the observational noise in order to be used to perform unbiased Bayesian parameter inference. To this end, we introduce -- a long short-term memory (LSTM) NN emulator of the global 21 cm signal that leverages the intrinsic correlation between frequency channels to achieve exceptional accuracy compared to previous emulators, which are all feedforward, fully connected NNs. LSTM NNs are a type of recurrent NN designed to capture long-term dependencies in sequential data. When trained and tested on the same simulated set of global 21 cm signals as the best previous emulators, has average relative rms error of 0.22% -- equivalently 0.39 mK -- and comparably fast evaluation time. We perform seven-dimensional Bayesian parameter estimation analyses using to fit global 21 cm signal mock data with different adopted observational noise levels, . The posterior rms error is less than for each fit and consistently decreases for tighter noise levels, showing that can sufficiently exploit even very optimistic measurements of the global 21 cm signal. We made the emulator, code, and data sets publicly available so that can be independently tested and used to retrain and constrain other 21 cm models.
... These used radio interferometers including the ★ E-mail: sudiptas@mail.tau.ac.il Low Frequency Array (LOFAR, Gehlot et al. 2019), the Murchison Wide-field Array (MWA, Trott et al. 2020), the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA, Eastwood et al. 2019), the Large-aperture Experiment to detect the Dark Age (LEDA, Price et al. 2018;Garsden et al. 2021), the New Extention in Nançay Upgrading LOFAR (NenuFAR, Mertens et al. 2021), and the Hydrogen Epoch of Reionization Array (HERA, DeBoer et al. 2017;Abdurashidova et al. 2022b). The upcoming Square Kilometre Array (SKA, Koopmans et al. 2015) is expected to provide measurements of the 21-cm power spectrum over a wide range of redshifts including cosmic dawn. ...
Several ongoing and upcoming radio telescopes aim to detect either the global 21-cm signal or the 21-cm power spectrum. The extragalactic radio background, as detected by ARCADE-2 and LWA-1, suggests a strong radio background from cosmic dawn, which can significantly alter the cosmological 21-cm signal, enhancing both the global signal amplitude and the 21-cm power spectrum. In this paper, we employ an artificial neural network (ANN) to check if there is a radio excess over the Cosmic Microwave Background (CMB) in mock data, and if present, we classify its type into one of two categories, a background from high-redshift radio galaxies or a uniform exotic background from the early Universe. Based on clean data (without observational noise), the ANN can predict the background radiation type with accuracy for the power spectrum and for the global signal. Although observational noise reduces the accuracy, the results remain quite useful. We also apply ANNs to map the relation between the 21-cm power spectrum and the global signal. By reconstructing the global signal using the 21-cm power spectrum, an ANN can estimate the global signal range consistent with an observed power spectrum from SKA-like experiments. Conversely, we show that an ANN can reconstruct the 21-cm power spectrum over a wide range of redshifts and wavenumbers given the global signal over the same redshifts. Such trained networks can potentially serve as a valuable tool for cross-confirmation of the 21-cm signal.
... The unique physical information contained in this signal has led to the development of an entire field known as 21-cm cosmology (e.g., Barkana & Loeb 2001;Furlanetto et al. 2006;Pritchard & Loeb 2012;Mesinger 2019). Forecasts for experiments such as the GMRT (Paciga et al. 2013), LOFAR (Patil et al. 2017;Mertens et al. 2020;Ghara et al. 2020), NenuFAR (Mertens et al. 2021;Munshi et al. 2024), LWA (Eastwood et al. 2019), MWA (Barry et al. 2019;Li et al. 2019;Trott et al. 2020), PAPER (Kolopanis et al. 2019), HERA (DeBoer et al. 2017;Abdurashidova et al. 2022), and SKA Mellema et al. 2015) aim to significantly improve constraints on the models describing the astrophysics of the first ionizing sources as well as cosmological parameters (Liu & Shaw 2020). ...
The 21-cm brightness-temperature field of neutral hydrogen during the Epoch of Reionization and Cosmic Dawn is a rich source of cosmological and astrophysical information, primarily due to its significant non-Gaussian features. However, the complex, nonlinear nature of the underlying physical processes makes analytical modelling of this signal challenging. Consequently, studies often resort to semi-numerical simulations. Traditional analysis methods, which rely on a limited set of summary statistics, may not adequately capture the non-Gaussian content of the data, as the most informative statistics are not predetermined. This paper explores the application of machine learning (ML) to surpass the limitations of summary statistics by leveraging the inherent non-Gaussian characteristics of the 21-cm signal. We demonstrate that a well-trained neural network can independently reconstruct the hydrogen density, spin-temperature, and neutral-fraction fields with cross-coherence values exceeding 0.95 for k-modes below 0.5 Mpc h, based on a representative simulation at a redshift of . To achieve this, the neural network utilises the non-Gaussian information in brightness temperature images over many scales. We discuss how these reconstructed fields, which vary in their sensitivity to model parameters, can be employed for parameter inference, offering more direct insights into underlying cosmological and astrophysical processes only using limited summary statistics of the brightness temperature field, such as its power spectrum.
... Probing EoR directly using the redshifted 21-cm signal with the current instruments is also challenging because of several hindrances such as large (∼ 10 4 times) foregrounds (e.g., Ali et al. 2008;Bernardi, G. et al. 2009Bernardi, G. et al. , 2010Ghosh et al. 2012), thermal noise, radio frequency interference, ionospheric turbulence and other systematics. While no undisputed detection of the EoR 21-cm signal has been achieved so far, the current data from the radio-interferometric experiments have been able to provide a few upper limits on the EoR 21-cm power spectra (e.g., LOFAR: Patil et al. 2017;Mertens et al. 2020, MWA: Barry et al. 2019Li et al. 2019;Trott et al. 2020, HERA: Abdurashidova et al. 2022The HERA Collaboration et al. 2022), and the upperlimits are improving gradually. ...
We propose different estimators to probe the epoch of reionization (EoR) intergalactic medium (IGM) using the dispersion measure () of the FRBs. We consider three different reionization histories which we can distinguish with a total of measurements during EoR if their redshifts are known. We note that the redshift derivatives of are also directly sensitive to the reionization history. The major point of this work is exploring the variance in the measurements and the information encoded in them. We find that the all-sky average gets biased from the LoS fluctuations in the measurements introduced by the ionization of IGM during EoR. We find that the ratio depends directly on the ionization bubble sizes as well as the reionization history. On the other hand, we also find that angular variance (coined as structure function) of encodes the information about the duration of reionization and the typical bubble sizes as well. We establish the usefulness of variances in using toy models of reionization and later verify it with the realistic reionization simulations.
... Mertens et al. 2020;Ghara et al. 2020), Murchison Widefield Array (MWA; e.g. Tingay et al. 2013;Trott et al. 2020) and Hydrogen Epoch of Reionization Array (HERA; e.g. The HERA Collaboration et al. 2022;HERA Collaboration et al. 2023). ...
The low-frequency component of the upcoming Square Kilometre Array Observatory (SKA-Low) will be sensitive enough to construct 3D tomographic images of the 21-cm signal distribution during reionization. However, foreground contamination poses challenges for detecting this signal, and image recovery will heavily rely on effective mitigation methods. We introduce \texttt{SERENEt}, a deep-learning framework designed to recover the 21-cm signal from SKA-Low's foreground-contaminated observations, enabling the detection of ionized (HII) and neutral (HI) regions during reionization. \texttt{SERENEt} can recover the signal distribution with an average accuracy of 75 per cent at the early stages () and up to 90 per cent at the late stages of reionization (). Conversely, HI region detection starts at 92 per cent accuracy, decreasing to 73 per cent as reionization progresses. Beyond improving image recovery, \texttt{SERENEt} provides cylindrical power spectra with an average accuracy exceeding 93 per cent throughout the reionization period. We tested \texttt{SERENEt} on a 10-degree field-of-view simulation, consistently achieving better and more stable results when prior maps were provided. Notably, including prior information about HII region locations improved 21-cm signal recovery by approximately 10 per cent. This capability was demonstrated by supplying \texttt{SERENEt} with ionizing source distribution measurements, showing that high-redshift galaxy surveys of similar observation fields can optimize foreground mitigation and enhance 21-cm image construction.
... Without producing actual images, CHIPS uses least-squares spectral analysis to transform the LOS axis from frequency into k-space. The power spectrum is then estimated with a maximum likelihood estimator (Trott et al. 2020). DOM image power spectrum is fundamentally different from the existing methods in two ways: DOM maps visibilities at their original uv locations without gridding Barry et al. 2019;Offringa et al. 2019); the DOM mapping process is a simple linear operation without iteration (Högbom 1974;Clark 1980;Cornwell 2008;Rau & Cornwell 2011;Mertens et al. 2020). ...
The key to detecting neutral hydrogen during the epoch of reionization (EoR) is to separate the cosmological signal from the dominating foreground radiation. We developed direct optimal mapping (DOM) to map interferometric visibilities; it contains only linear operations, with full knowledge of point spread functions from visibilities to images. Here, we demonstrate a fast Fourier transform-based image power spectrum and its window functions computed from the DOM images. We use noiseless simulation, based on the Hydrogen Epoch of Reionization Array Phase I configuration, to study the image power spectrum properties. The window functions show <10 ⁻¹¹ of the integrated power leaks from the foreground-dominated region into the EoR window; the 2D and 1D power spectra also verify the separation between the foregrounds and the EoR.
We study the reionization of the Universe due to haloes that host galaxies undergoing bursts of star formation. By comparing the recent results from the James Webb Space Telescope (JWST) with the cosmological hydrodynamical simulation eagle at z ≥ 6, we find that bursty galaxies have specific star formation rate, sSFR > 10⁻² Myr⁻¹, and magnitude, M UV ≤ -17. Most of them reside in haloes of mass ∼ 10⁹ M⊙ and some in more massive haloes. We then construct the models of escape fraction and find that a skewed Gaussian function with a flat tail towards the high mass end best describes the mean dependence of escape fraction on halo mass, considering the haloes hosting bursty galaxies as the primary drivers of reionization. We implement the models of escape fraction in the code 21cmfast to study the progress of reionization and derive the evolution of the mean ionized fraction that agrees well with observations. We also calculate the brightness temperature, spin temperature, and kinetic temperature and further study the spatial fluctuations in these quantities to gain insights into the progress of reionization. We compute the 21 cm power spectrum and predict a peak in power at 180 MHz corresponding to redshift, z ≈ 6.8, that is testable by the upcoming Square Kilometre Array (SKA). Our findings suggest that the Universe was reionized by the haloes of ≳ 10⁹ M⊙.
The redshifted 21 cm signal of neutral hydrogen can be used as a direct probe of the intergalactic medium during Cosmic Dawn (CD) and Epoch of Reionization (EoR). However, detecting this inherently weak signal has numerous challenges. The major ones include accurate foreground removal from low-frequency radio observations and systematics arising from instrumental effects. The Earth's ionosphere poses a major obstacle at these low radio frequencies. Thus, a systematic study of ionospheric effects on these sensitive low-frequency observations is critical, given that the construction of the Square Kilometre Array (SKA1-Low) is in full progress. We use the end-to-end pipeline, called 21cmE2E, to study the effect of time-varying ionospheric corruption on the 21 cm power spectrum recovery. We use two models: a) a catalogue-based model focused on source position shift due to the refractive effect of the ionosphere and b) a realistic ionospheric condition generated using Kolmogorov's turbulence model. We assess the effect of the imperfections thus generated on the extraction of Hi 21 cm signal power spectrum. Our study shows that beyond “median ionospheric offset” (θ MIO ≲ 0.1”), the 21 cm signal from the EoR is unaffected by residual ionospheric effects. Our study emphasizes the need for the development of efficient ionospheric calibration algorithms for the upcoming SKA1-Low observations to extract the Hi 21 cm power spectra from the CD/EoR.
Simulating Population (Pop.) III star formation in mini-halos in a large cosmological simulation is an extremely challenging task but it is crucial to estimate its impact on the 21cm power spectrum. In this work, we develop a framework within the semi-analytical code meraxes to estimate the radiative backgrounds from Pop. III stars needed for the computation of the 21cm signal. We computed the 21cm global signal and power spectrum for different Pop. III models varying star formation efficiency, initial mass function (IMF) and specific X-ray luminosity per unit of star formation (LX/SFR). In all the models considered, we find Pop. III stars have little to no impact on the reionization history but significantly affect the thermal state of the intergalactic medium (IGM) due to the strong injection of X-ray photons from their remnants that heat the neutral IGM at 15. This is reflected not only on the 21cm sky-averaged global signal during the Cosmic Dawn but also on the 21cm power spectrum at 10 where models with strong Pop. III X-ray emission have larger power than models with no or mild Pop. III X-ray emission. We estimate observational uncertainties on the power spectrum using 21cmsense and find that models where Pop. III stars have a stronger X-ray emission than Pop. II are distinguishable from models with no or mild Pop. III X-ray emission with 1000 hours observations of the upcoming SKA1-low.
The precise characterization and mitigation of systematic effects is one of the biggest roadblocks impeding the detection of the fluctuations of cosmological 21 cm signals. Missing data in radio cosmological experiments, often due to radio frequency interference (RFI), pose a particular challenge to power spectrum analysis as this could lead to the ringing of bright foreground modes in the Fourier space, heavily contaminating the cosmological signals. Here we show that the problem of missing data becomes even more arduous in the presence of systematic effects. Using a realistic numerical simulation, we demonstrate that partially flagged data combined with systematic effects can introduce significant foreground ringing. We show that such an effect can be mitigated through inpainting the missing data. We present a rigorous statistical framework that incorporates the process of inpainting missing data into a quadratic estimator of the 21 cm power spectrum. Under this framework, the uncertainties associated with our inpainting method and its impact on power spectrum statistics can be understood. These results are applied to the latest Phase II observations taken by the Hydrogen Epoch of Reionization Array, forming a crucial component in power spectrum analyses as we move toward detecting 21 cm signals in the ever more noisy RFI environment.
The 21-cm forest signal is a promising probe of the Epoch of Reionization complementary to other 21-cm line observables and Lyα forest signal. Prospects of detecting it have significantly improved in the last decade thanks to the discovery of more than 30 radio-loud quasars at these redshifts, upgrades to telescope facilities, and the notion that neutral hydrogen islands persist down to z ≲ 5.5. We forward-model the 21-cm forest signal using semi-numerical simulations and incorporate various instrumental features to explore the potential of detecting the 21-cm forest at z = 6, both directly and statistically, with the currently available (uGMRT) and forthcoming (SKA1-low) observatories. We show that it is possible to detect the 1D power spectrum of the 21-cm forest spectrum, especially at large scales of with the of the uGMRT time and with the SKA1-low over if the intergalactic medium (IGM) is 25% neutral and these neutral hydrogen regions have a spin temperature of . On the other hand, we infer that a null-detection of the signal with such observations of 10 radio-loud sources at z ≈ 6 can be translated into constraints on the thermal and ionization state of the IGM which are tighter than the currently available measurements. Moreover, a null-detection of the 1D 21-cm forest power spectrum with only of the uGMRT observations of 10 radio-loud sources can already be competitive with the Lyα forest and 21-cm tomographic observations in disfavouring models of significantly neutral and cold IGM at z = 6.
The final stages of cosmic reionisation (EndEoR) are expected to be strongly regulated by the residual neutral hydrogen in the already ionised regions of the Universe. Its presence limits the mean distance that ionising photons can travel and hence, the extent of the regions that sources of ionising photons can affect. The structures containing most of this residual neutral hydrogen are typically unresolved in large-scale simulations of reionisation. Here, we investigate and compare a range of approaches for including the effect of these small-scale absorbers, also known as Lyman limit systems (LLS), in such simulations. We evaluate the impact of these different approaches on the reionisation history, the evolution of the ultraviolet background, and its fluctuations. We also compare to observational results on the distribution of Lyman-α opacity towards the EndEoR and the measured mean free path of ionising photons. We further consider their effect on the 21-cm power spectrum. We find that although each of the different approaches can match some of the observed probes of the final stages of reionisation, only the use of a redshift-dependent and position-dependent LLS model is able to reproduce all of them. We therefore recommend that large-scale reionisation simulations, which aim to describe both the state of the ionised and neutral intergalactic medium use such an approach, although the other, simpler approaches are applicable depending on the science goal of the simulation.
One of the main obstacles preventing the detection of the redshifted 21 cm signal from neutral hydrogen in the early Universe is the astrophysical foreground emission, which is several orders of magnitude brighter than the signal. The foregrounds, due to their smooth spectra, are expected to predominantly occupy a region in the cylindrical power spectrum known as the foreground wedge. However, the conventional equations describing the extent of the foreground wedge are derived under a flat-sky approximation. This assumption breaks down for tracking wide-field instruments, thus rendering these equations inapplicable in these situations. In this paper we derive equations for the full sky foreground wedge, and show that the foregrounds can potentially extend far beyond what the conventional equations suggest. We also derive the equations that describe a specific bright source in the cylindrical power spectrum space. The validity of both sets of equations is tested against numerical simulations. Many current and upcoming interferometers (e.g., LOFAR, NenuFAR, MWA, SKA) are wide-field phase-tracking instruments. These equations give us new insights into the nature of foreground contamination in the cylindrical power spectra estimated using wide-field instruments. Additionally, they allow us to accurately associate features in the power spectrum with foregrounds or instrumental effects. The equations are also important for correctly selecting the epoch of reionization (EoR) window for foreground avoidance analyses, and for planning 21 cm observations. In future analyses, we recommend using these updated horizon lines to indicate the foreground wedge in the cylindrical power spectrum accurately. The new equations for generating the updated wedge lines are made available in the Python library pslines
Context. Current radio interferometers output multi-petabyte-scale volumes of data per year, making the storage, transfer, and processing of these data a sizeable challenge. This challenge is expected to grow with next-generation telescopes such as the Square Kilometre Array (SKA), which will produce a considerably larger data volume than current instruments. Lossy compression of interferometric data post-correlation can abate this challenge, but any drawbacks from the compression should be well understood in advance.
Aims. Lossy data compression reduces the precision of data, introducing additional noise. Since high-redshift (e.g., cosmic dawn or epoch of reionization) 21 cm studies impose strict precision requirements, the impact of this effect on the 21 cm signal power spectrum statistic is investigated in a bid to rule out unwanted systematics.
Methods. We applied DYSCO visibility compression, a technique for normalizing and quantizing specifically designed for radio interferometric data, to observed visibilities datasets from the LOFAR telescope as well as simulated ones. The power spectrum of these data was analyzed, and we establish the level of the compression noise in the power spectrum in comparison to the thermal noise. We also examined its coherency behavior by employing the cross-coherence metric. Finally, for optimal compression results, we compared the compression noise obtained from different compression settings to a nominal 21 cm signal power.
Results. From a single night of observation, we find that the noise introduced due to the compression is more than five orders of magnitude lower than the thermal noise level in the power spectrum. The noise does not affect calibration. Furthermore, the noise remains subdominant to the noise introduced by the nonlinear calibration algorithm used following random parameter initialization across different runs. The compression noise shows no correlation with the sky signal and has no measurable coherent component, therefore averaging down optimally with the integration of more data. The level of compression error in the power spectrum ultimately depends on the compression settings.
Conclusions. DYSCO visibility compression is found to be an insignificant concern for 21 cm power spectrum studies. Hence, data volumes can be safely reduced by factors of ∼4 with insignificant bias to the final power spectrum. Data from SKA-Low will likely be compressible by the same factor as data from LOFAR owing to the similarities of the two instruments. The same technique can be used to compress data from other telescopes, but a small adjustment of the compression parameters might be required.
The Generalized Needlet Internal Linear Combination (GNILC) method is a non-parametric component separation algorithm to remove the foreground contamination of the 21-cm intensity mapping data. In this work, we perform the Discrete Cosine Transform (DCT) along the frequency axis in the expanded GNILC framework (denoted eGNILC) which helps reduce the power loss in low multipoles, and further demonstrate its performance. We also calculate the eGNILC bias to modify the criterion for determining the degrees of freedom of the foreground (dof), and embed the Robust Principal Component Analysis (RPCA) in mixing matrix computation to obtain a blind component separation method. We find that the eGNILC bias is related to the averaged domain size and the dof of the foreground but not the underlying 21-cm signal. In case of no beam effect, the eGNILC bias is negligible for simple power law foregrounds outside the Galactic plane. We also examine the eGNILC performance in the SKA-MID (SKA Phase-I in mid-frequency) and BINGO (Baryon Acoustic Oscillations from Integrated Neutral Gas Observations) simulations. We show that if the adjacent frequency channels are not highly correlated, eGNILC can recover the underlying 21-cm signal with good accuracy. With the varying Airy disk beam applied to both SKA-MID and BINGO, the power spectra of 21-cm can be effectively recovered at the multipoles and respectively. With no instrumental noise, the SKA-MID exhibits power loss and BINGO exhibits power loss. The varying Airy-disk beam only causes significant errors at large multipoles.
We study the viability of using power spectrum clustering wedges as summary statistics of 21 cm surveys during the Epoch of Reionization (EoR). For observations in a wide redshift range z ∼ 7 − 9 corresponding to a line-of-sight scale of ∼500 Mpc, the power spectrum is subject to anisotropic effects due to the evolution along the light-of-sight. Information on the physics of reionization can be extracted from the anisotropy using the power spectrum multipoles. Signals of the power spectrum monopole are highly correlated at scales smaller than the typical ionization bubble, which can be disentangled by including higher-order multipoles. By simulating observations of the low frequency part of the Square Kilometre Array (SKA) Observatory, we find that the sampling of the cylindrical wavenumber k-space is highly non-uniform due to the baseline distribution, i.e. the distribution of antenna pairs sampling different transverse k⊥ scales. Measurements in clustering wedges partition the cylindrical k-space into different radial k∥ scales, and can be used for isolating parts of k-space with relatively uniform sampling, allowing for more precise parameter inference. Using Fisher Matrix forecasts, we find that the reionization model can be inferred with per-cent level precision with ∼120 hrs of integration time using SKA-Low. Compared to model inference using only the power spectrum monopole above the foreground wedge, model inference using multipole power spectra in clustering wedges yields a factor of ∼3 improvement on the marginalised 1D parameter constraints.
Power spectrum estimators are an important tool in efforts to detect the 21 cm brightness temperature fluctuations from neutral hydrogen at early times. An initial detection will likely be statistical in nature, meaning that it will not be possible to make a coherent map of the brightness temperature fluctuations; instead, only their variance will be measured against a background of noise and residual systematic effects. Optimal Quadratic Estimator (OQE)-based methods often apply an inverse covariance weighting to the data. However, inaccurate covariance modelling can lead to reduced sensitivity and, in some cases, severe signal loss. We recently proposed a Bayesian method to jointly estimate the 21 cm fluctuations, their power spectrum, and foreground emission. Instead of requiring a fixed a priori estimate of the covariance, we estimate the covariance as part of the inference. Choices of parametrization, particularly of the foregrounds, are subject to model errors and could lead to biases and other ill effects if not properly controlled. In this paper, we investigate the effects of inaccurate foreground models on 21 cm power spectrum recovery. Using simulated visibilities, we find that, even in the most extreme scenarios tested, our approach is capable of recovering 21 cm delay power spectrum estimates consistent with a known input signal for delays ≳ 300 ns (∼88% of the available Fourier modes). This is true even when using foreground models derived from modified foreground catalogs containing spatial and spectral perturbations at the quoted level of uncertainty on our foreground catalogs.
The Minkowski functionals (MFs), a set of topological summary statistics, have emerged as a powerful tool for extracting non-Gaussian information. We investigate the prospect of constraining the reionization parameters using the MFs of the 21 cm brightness temperature field from the epoch of reionization (EOR). Realistic effects, including thermal noise, synthesized beam, and foreground avoidance, are applied to the mock observations from radio interferometric array experiments such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA). We demonstrate that the MFs of the 21 cm signal measured with SKA-Low can be used to distinguish different reionization models, whereas the MF measurement with a HERA-like array cannot be made accurately enough. We further forecast the accuracies with which the MF measurements can place constraints on reionization parameters, using the standard Markov Chain Monte Carlo analysis for parameter inference based on forward modeling. We find that for SKA-Low observation, MFs provide unbiased estimations of the reionization parameters with accuracies comparable to the power spectrum (PS) analysis. Furthermore, joint constraints using both MFs and PS can improve the constraint accuracies by up to 30% compared to those with the PS alone. Nevertheless, the constraint accuracies can be degraded if the EOR window is shrunk with strong foreground avoidance. Our analysis demonstrates the promise of MFs as a set of summary statistics that extract complementary information from the 21 cm EOR field to the two-point statistics, which suggests a strong motivation for incorporating the MFs into the data analysis of future 21 cm observations.
The morphology of the 21-cm signal emitted by the neutral hydrogen present in the intergalactic medium (IGM) during the Epoch of Reionization (EoR) depends both on the properties of the sources of ionizing radiation and on the underlying physical processes within the IGM. Variation in the morphology of the IGM 21-cm signal due to the different sources of the EoR is expected to have a significant impact on the 21-cm bispectrum, which is one of the crucial observable statistics that can evaluate the non-Gaussianity present in the signal and which can be estimated from radio interferometric observations of the EoR. Here we present the 21-cm bispectrum for different reionization scenarios assuming different simulated models for the sources of reionization. We also demonstrate how well the 21-cm bispectrum can distinguish between different IGM 21-cm signal morphologies, arising due to the differences in the reionization scenarios, which will help us shed light on the nature of the sources of ionizing photons. Our estimated large-scale bispectrum for all unique k-triangle shapes shows a significant difference in the magnitude and sign across different reionization scenarios. Additionally, our focused analysis of bispectrum for a few specific k-triangle shapes (e.g. squeezed-limit, linear, and shapes in the vicinity of the squeezed-limit) shows that the large scale 21-cm bispectrum can distinguish between reionization scenarios that show inside-out, outside-in and a combination of inside-out and outside-in morphologies. These results highlight the potential of using the 21-cm bispectrum for constraining different reionization scenarios.
A first statistical detection of the 21-cm Epoch of Reionization (EoR) is on the horizon, as cosmological volumes of the Universe become accessible via the adoption of low-frequency interferometers. We explore the impact which non-identical instrumental beam responses can have on the calibrated power spectrum and a future EoR detection. All-sky satellite measurements of Murchison Widefield Array (MWA) beams have revealed significant sidelobe deviations from cutting-edge electromagnetic simulations at the zenith power level. By generating physically motivated deformed beam models, we emulate real measurements of the MWA which inherently encode the imprints of varied beams. We explore two calibration strategies: using a single beam model across the array, or using a full set of deformed beams. Our simulations demonstrate beam-induced leakage of foreground power into theoretically uncontaminated modes, at levels which exceed the expected cosmological signal by factors of over ∼1000 between the modes k=0.1-1 hMpc−1. We also show that this foreground leakage can be mitigated by including measured models of varied beams into calibration frameworks, reducing the foreground leakage to a sub-dominant effect and potentially unveiling the EoR. Finally, we outline the future steps necessary to make this approach applicable to real measurements by radio interferometers.
A first statistical detection of the 21-cm Epoch of Reionization (EoR) is on the horizon, as cosmological volumes of the Universe become accessible via the adoption of low-frequency interferometers. We explore the impact which non-identical instrumental beam responses can have on the calibrated power spectrum and a future EoR detection. All-sky satellite measurements of Murchison Widefield Array (MWA) beams have revealed significant sidelobe deviations from cutting-edge electromagnetic simulations at the ~10% zenith power level. By generating physically motivated deformed beam models, we emulate real measurements of the MWA which inherently encode the imprints of varied beams. We explore two calibration strategies: using a single beam model across the array, or using a full set of deformed beams. Our simulations demonstrate beam-induced leakage of foreground power into theoretically uncontaminated modes, at levels which exceed the expected cosmological signal by factors of over ~1000 between the modes k=0.1-1 . We also show that this foreground leakage can be mitigated by including measured models of varied beams into calibration frameworks, reducing the foreground leakage to a sub-dominant effect and potentially unveiling the EoR. Finally, we outline the future steps necessary to make this approach applicable to real measurements by radio interferometers.
During the epoch of reionization (EoR), the 21-cm signal allows direct observation of the neutral hydrogen (H i) in the intergalactic medium (IGM). In the post-reionization era, this signal instead probes H i in galaxies, which traces the dark matter density distribution. With new numerical simulations, we investigated the end stages of reionization to elucidate the transition of our Universe into the post-reionization era. Our models are consistent with the latest high-redshift measurements, including ultraviolet (UV) luminosity functions up to redshift 8. Notably, these models consistently reproduced the evolution of the UV photon background, which is constrained from Lyman- absorption spectra. We studied the dependence of this background on the nature of photon sinks in the IGM, requiring mean free path of UV photons to be 10 comoving-megaparsecs (cMpc) during the EoR that increases gradually with time during late stages (). Our models revealed that the reionization of the IGM transitioned from an inside-out to an outside-in process when the Universe is less than 0.01 per cent neutral. During this epoch, the 21-cm signal also shifted from probing predominantly the H i in the IGM to that in galaxies. Furthermore, we identified a statistically significant number of large neutral islands (with sizes up to 40 cMpc) persisting until very late stages () that can imprint features in Lyman- absorption spectra and also produce a knee-like feature in the 21-cm power spectrum.
The Epoch of Reionization (EoR) is a crucial link through which the complete evolutionary history of the Universe can be grasped. Several attempts with a variety of observables have been used to understand the thermal and ionization evolution of the intergalactic medium during EoR. In this study, we explore the simultaneous prospects of two important observables that are expected to be available in the near future, that is, the dispersion measure (DM) of high-redshift fast radio bursts (FRBs) and large-scale 21 cm power spectra. For this purpose, we used a previously developed explicitly photon conserving seminumerical model SCRIPT which includes realistic recombination and radiative feedback effects. We verified that the DM evolution of 100 mock FRBs at high redshift (k h/ cMpc 8 $ for the reionization interval and midpoint at the 95 confidence), alleviate the degeneracies, and can narrow down the thermal history of the Universe by discarding some of the extreme heating models.
Detecting the 21-cm signal at z ≳ 6 will reveal insights into the properties of the first galaxies responsible for driving reionisation. To extract this information, we perform parameter inference with 3D simulations of the 21-cm signal embedded within a Bayesian inference pipeline. Presently, when performing inference, we must choose which sources of uncertainty to sample and which to hold fixed. Since the astrophysics of galaxies is much more uncertain than that of the underlying halo-mass function (HMF), we typically parameterise and model the former while fixing the latter. However, doing so may bias our inference of the galaxy properties. In this work, we explore the consequences of assuming an incorrect HMF and quantify the relative biases on our inferred astrophysical model parameters when considering the wrong HMF. We then relax this assumption by constructing a generalised five parameter HMF model and simultaneously recover it with our underlying astrophysical model. For this, we use 21CMFAST and perform Simulation-Based Inference using marginal neural ratio estimation to learn the likelihood-to-evidence ratio with Swyft. Using a mock 1000-hour observation of the 21-cm power spectrum from the forthcoming Square Kilometre Array, conservatively assuming foreground wedge avoidance, we find that assuming the incorrect HMF can bias the recovered astrophysical parameters by up to ∼3 − 4σ even when including independent information from observed luminosity functions. Using our generalised HMF model, although we recover our astrophysical parameters with a factor of ∼2 − 4 larger marginalised uncertainties, the constraints are unbiased, agnostic to the underlying HMF and therefore more conservative.
The turbulent ionosphere causes phase shifts to incoming radio waves on a broad range of temporal and spatial scales. When an interferometer is not sufficiently calibrated for the direction-dependent ionospheric effects, the time-varying phase shifts can cause the signal to decorrelate. The ionosphere’s influence over various spatiotemporal scales introduces a baseline-dependent effect on the interferometric array. We study the impact of baseline-dependent decorrelation on high-redshift observations with the Low Frequency Array (LOFAR). Datasets with a range of ionospheric corruptions are simulated using a thin-screen ionosphere model, and calibrated using the state-of-the-art LOFAR Epoch of Reionisation pipeline. For the first time ever, we show the ionospheric impact on various stages of the calibration process including an analysis of the transfer of gain errors from longer to shorter baselines using realistic end-to-end simulations. We find that direction-dependent calibration for source subtraction leaves excess power of up to two orders of magnitude above the thermal noise at the largest spectral scales in the cylindrically averaged auto-power spectrum under normal ionospheric conditions. However, we demonstrate that this excess power can be removed through Gaussian process regression, leaving no excess power above the ten per cent level for a 5 km diffractive scale. We conclude that ionospheric errors, in the absence of interactions with other aggravating effects, do not constitute a dominant component in the excess power observed in LOFAR Epoch of Reionisation observations of the North Celestial Pole. Future work should therefore focus on less spectrally smooth effects, such as beam modelling errors.
Predicting the 21-cm signal from the epoch of reionization and cosmic dawn is a complex and challenging task. Various simplifying assumptions have been applied over the last decades to make the modeling more affordable. In this paper, we investigate the validity of several such assumptions, using a simulation suite consisting of three different astrophysical source models that agree with the current constraints on the reionization history and the UV luminosity function. We first show that the common assumption of a saturated spin temperature may lead to significant errors in the 21-cm clustering signal over the full reionization period. The same is true for the assumption of a neutral universe during the cosmic dawn which may lead to significant deviation from the correct signal during the heating and the Lyman-α coupling period. Another popular simplifying assumption consists of predicting the global differential brightness temperature (dTb) based on the average quantities of the reionization fraction, gas temperature, and Lyman-α coupling. We show that such an approach leads to a 10 percent deeper absorption signal compared to the results obtained by averaging the final dTb-map. Finally, we investigate the simplifying method of breaking the 21-cm clustering signal into different auto and cross components that are then solved assuming linearity. We show that even though the individual fields have a variance well below unity, they often cannot be treated perturbatively as the perturbations are strongly non-Gaussian. As a consequence, predictions based on the perturbative solution of individual auto and cross power spectra may lead to strongly biased results, even if higher-order terms are taken into account.
The turbulent ionosphere causes phase shifts to incoming radio waves on a broad range of temporal and spatial scales. When an interferometer is not sufficiently calibrated for the direction-dependent ionospheric effects, the time-varying phase shifts can cause the signal to decorrelate. The ionosphere's influence over various spatiotemporal scales introduces a baseline-dependent effect on the interferometric array. We study the impact of baseline-dependent decorrelation on high-redshift observations with the Low Frequency Array (LOFAR). Datasets with a range of ionospheric corruptions are simulated using a thin-screen ionosphere model, and calibrated using the state-of-the-art LOFAR Epoch of Reionisation pipeline. For the first time ever, we show the ionospheric impact on various stages of the calibration process including an analysis of the transfer of gain errors from longer to shorter baselines using realistic end-to-end simulations. We find that direction-dependent calibration for source subtraction leaves excess power of up to two orders of magnitude above the thermal noise at the largest spectral scales in the cylindrically averaged auto-power spectrum under normal ionospheric conditions. However, we demonstrate that this excess power can be removed through Gaussian process regression, leaving no excess power above the ten per cent level for a km diffractive scale. We conclude that ionospheric errors, in the absence of interactions with other aggravating effects, do not constitute a dominant component in the excess power observed in LOFAR Epoch of Reionisation observations of the North Celestial Pole. Future work should therefore focus on less spectrally smooth effects, such as beam modelling errors.
Early results from the JWST observations have reported a surprisingly high number of UV-bright galaxies at z ≥ 10, which appears to challenge the theoretical predictions from standard galaxy formation models in the ΛCDM framework at these redshifts. To alleviate this tension, several cosmological and astrophysical interpretations have been advanced. However, all of these proposed scenarios carry noteworthy consequences for other large-scale processes in the early Universe, particularly cosmic reionization, since high-redshift galaxies are believed to be the primary ionizing sources during the Epoch of Reionization (EoR). To investigate this, we introduce a semi-analytical model of galaxy formation and evolution that explains the evolving galaxy UV luminosity function (UVLF) over 6 ≲ z ≲ 15, and also jointly tracks the time evolution of the globally averaged neutral hydrogen fraction in the intergalactic medium. The model self-consistently accounts for the suppression of star formation in low-mass galaxies due to reionization feedback and is constrained by comparing the model predictions with various observational probes like the UVLF data from HST and JWST, recent measurements of the neutral hydrogen fraction, and the CMB scattering optical depth. Our analysis confirms that a rapid enhancement in the star-formation rate efficiency and/or UV luminosity per stellar mass formed is necessary for consistency with the JWST UVLF estimates at z ≥ 10. We further find that it is possible to jointly satisfy the current reionization constraints when the escape fraction is assumed to be halo-mass dependent, requiring higher Lyman-continuum leakage from low-mass galaxies. We also examine the relative contribution of galaxies with different UV luminosities towards the ionizing photon budget for the EoR and investigate the large-scale bias of high-z galaxies.
Measurements of 21 cm Epoch of Reionization (EoR) structure are subject to systematics originating from both the analysis and the observation conditions. Using 2013 data from the Murchison Widefield Array (MWA), we show the importance of mitigating both sources of contamination. A direct comparison between results from Beardsley et al. and our updated analysis demonstrates new precision techniques, lowering analysis systematics by a factor of 2.8 in power. We then further lower systematics by excising observations contaminated by ultra-faint RFI, reducing by an additional factor of 3.8 in power for the zenith pointing. With this enhanced analysis precision and newly developed RFI mitigation, we calculate a noise-dominated upper limit on the EoR structure of Δ2 ≤ 3.9 × 103 mK2 at k = 0.20 h Mpc-1 and z = 7 using 21 hr of data, improving previous MWA limits by almost an order of magnitude.
The 21 cm transition of neutral hydrogen is opening an observational window into the cosmic dawn of the universe---the epoch of first star formation. We use 28 hr of data from the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) to place upper limits on the spatial power spectrum of 21 cm emission at z ≈ 18.4 (Δ_(21) ≲ 10^4mK), and within the absorption feature reported by the EDGES experiment (Bowman et al. 2018). In the process we demonstrate the first application of the double Karhunen-Loève transform for foreground filtering, and diagnose the systematic errors that are currently limiting the measurement. We also provide an updated model for the angular power spectrum of low-frequency foreground emission measured from the northern hemisphere, which can be used to refine sensitivity forecasts for next-generation experiments.
Structure imprinted in foreground extragalactic point sources by ionospheric refraction has the potential to contaminate Epoch of Reionization (EoR) power spectra of the 21 cm emission line of neutral hydrogen. The alteration of the spatial and spectral structure of foreground measurements due to total electron content gradients in the ionosphere creates a departure from the expected sky signal. We present a general framework for understanding the signatures of ionospheric behavior in the 2D neutral hydrogen power spectrum measured by a low-frequency radio interferometer. Two primary classes of ionospheric behavior are considered, corresponding to dominant modes observed in Murchison Widefield Array (MWA) EoR data, namely, anisotropic structured wave behavior and isotropic turbulence. Analytic predictions for power spectrum bias due to this contamination are computed and compared with simulations. We then apply the ionospheric metric described in Jordan et al. to study the impact of ionospheric structure on MWA data, by dividing MWA EoR data sets into classes with good and poor ionospheric conditions, using sets of matched 30-minute observations from 2014 September. The results are compared with the analytic and simulated predictions, demonstrating the observed bias in the power spectrum when the ionosphere is active (displays coherent structures or isotropic turbulence). The analysis demonstrates that unless ionospheric activity can be quantified and corrected, active data should not be included in EoR analysis in order to avoid systematic biases in cosmological power spectra. When data are corrected with a model formed from the calibration information, bias reduces below the expected 21 cm signal level. Data are considered "quiet" when the median measured source position offsets are less than 10.″-15.″. © 2018. The American Astronomical Society. All rights reserved..
Observations of the redshifted 21-cm hyperfine line of neutral hydrogen from early phases of the Universe such as Cosmic Dawn and the Epoch of Reionization promise to open a new window onto the early formation of stars and galaxies. We present the first upper limits on the power spectrum of redshifted 21-cm brightness temperature fluctuations in the redshift range z = 19.8–25.2 (54–68 MHz frequency range) using 14 h of data obtained with the LOFAR-Low Band Antenna (LBA) array. We also demonstrate the application of a multiple pointing calibration technique to calibrate the LOFAR-LBA dual-pointing observations centred on the North Celestial Pole and the radio galaxy 3C220.3. We observe an unexplained excess of ∼30−50 per cent in Stokes / noise compared to Stokes V for the two observed fields, which decorrelates on ≳12 s and might have a physical origin. We show that enforcing smoothness of gain errors along frequency direction during calibration reduces the additional variance in Stokes I compared Stokes V introduced by the calibration on sub-band level. After subtraction of smooth foregrounds, we achieve a 2σ upper limit on the 21-cm power spectrum of Δ221<(14561mK)2 at k∼0.038hcMpc−1 and Δ221<(14886mK)2 at k∼0.038hcMpc−1 for the 3C220 and NCP fields respectively and both upper limits are consistent with each other. The upper limits for the two fields are still dominated by systematics on most k modes.
A host of new low-frequency radio telescopes seek to measure the 21 cm transition of neutral hydrogen from the early universe. These telescopes have the potential to directly probe star and galaxy formation at redshifts 20 ≳ z ≳ 7 but are limited by the dynamic range they can achieve against foreground sources of low-frequency radio emission. Consequently, there is a growing demand for modern, high-fidelity maps of the sky at frequencies below 200 MHz for use in foreground modeling and removal. We describe a new wide-field imaging technique for drift-scanning interferometers: Tikhonov-regularized m-mode analysis imaging. This technique constructs images of the entire sky in a single synthesis imaging step with exact treatment of wide-field effects. We describe how the CLEAN algorithm can be adapted to deconvolve maps generated by m-mode analysis imaging. We demonstrate Tikhonov-regularized m-mode analysis imaging using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) by generating eight new maps of the sky north of δ = -30° with 15′ angular resolution at frequencies evenly spaced between 36.528 and 73.152 MHz and ∼800 mJy beam⁻¹ thermal noise. These maps are a 10-fold improvement in angular resolution over existing full-sky maps at comparable frequencies, which have angular resolutions ≥2°. Each map is constructed exclusively from interferometric observations and does not represent the globally averaged sky brightness. Future improvements will incorporate total power radiometry, improved thermal noise, and improved angular resolution due to the planned expansion of the OVRO-LWA to 2.6 km baselines. These maps serve as a first step on the path to the use of more sophisticated foreground filters in 21 cm cosmology incorporating the measured angular and frequency structure of all foreground contaminants. © 2018. The American Astronomical Society. All rights reserved.
After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.
We detail new techniques for analysing ionospheric activity, using Epoch of Reionisation (EoR) datasets obtained with the Murchison Widefield Array (MWA), calibrated by the `Real-Time System' (RTS). Using the high spatial- and temporal-resolution information of the ionosphere provided by the RTS calibration solutions over 19 nights of observing, we find four distinct types of ionospheric activity, and have developed a metric to provide an `at a glance' value for data quality under differing ionospheric conditions. For each ionospheric type, we analyse variations of this metric as we reduce the number of pierce points, revealing that a modest number of pierce points is required to identify the intensity of ionospheric activity; it is possible to calibrate in real-time, providing continuous information of the phase screen. We also analyse temporal correlations, determine diffractive scales, examine the relative fractions of time occupied by various types of ionospheric activity, and detail a method to reconstruct the total electron content responsible for the ionospheric data we observe. These techniques have been developed to be instrument agnostic, useful for application on LOFAR and SKA-Low.
We present the discovery of nine quasars at identified in the Sloan Digital Sky Survey (SDSS) imaging data. This completes our survey of quasars in the SDSS footprint. Our final sample consists of 52 quasars at , including 29 quasars with mag selected from 11,240 deg of the SDSS single-epoch imaging survey (the main survey), 10 quasars with selected from 4223 deg of the SDSS overlap regions (regions with two or more imaging scans), and 13 quasars down to mag from the 277 deg in Stripe 82. They span a wide luminosity range of . This well-defined sample is used to derive the quasar luminosity function (QLF) at . After combining our SDSS sample with two faint ( mag) quasars from the literature, we obtain the parameters for a double power-law fit to the QLF. The bright-end slope of the QLF is well constrained to be . Due to the small number of low-luminosity quasars, the faint-end slope and the characteristic magnitude are less well constrained, with and mag. The spatial density of luminous quasars, parametrized as , drops rapidly from to 6, with . Based on our fitted QLF and assuming an IGM clumping factor of C=3, we find that the observed quasar population cannot provide enough photons to ionize the IGM at \% confidence. Quasars may still provide a significant fraction of the required photons, although much larger samples of faint quasars are needed for more stringent constraints on the quasar contribution to reionization.
The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to measure 21 cm emission from the primordial intergalactic medium (IGM) throughout cosmic reionization (), and to explore earlier epochs of our Cosmic Dawn (). During these epochs, early stars and black holes heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is designed to characterize the evolution of the 21 cm power spectrum to constrain the timing and morphology of reionization, the properties of the first galaxies, the evolution of large-scale structure, and the early sources of heating. The full HERA instrument will be a 350-element interferometer in South Africa consisting of 14-m parabolic dishes observing from 50 to 250 MHz. Currently, 19 dishes have been deployed on site and the next 18 are under construction. HERA has been designated as an SKA Precursor instrument. In this paper, we summarize HERA's scientific context and provide forecasts for its key science results. After reviewing the current state of the art in foreground mitigation, we use the delay-spectrum technique to motivate high-level performance requirements for the HERA instrument. Next, we present the HERA instrument design, along with the subsystem specifications that ensure that HERA meets its performance requirements. Finally, we summarize the schedule and status of the project. We conclude by suggesting that, given the realities of foreground contamination, current-generation 21 cm instruments are approaching their sensitivity limits. HERA is designed to bring both the sensitivity and the precision to deliver its primary science on the basis of proven foreground filtering techniques, while developing new subtraction techniques to unlock new capabilities. The result will be a major step toward realizing the widely recognized scientific potential of 21 cm cosmology.
We confirm our recent prediction of the "pitchfork" foreground signature in
power spectra of high-redshift 21 cm measurements, wherein the interferometer
is sensitive to large-scale structure on all baselines. This is due to the
inherent response of a wide-field instrument and is characterized by enhanced
power from foreground emission in Fourier modes adjacent to those considered to
be most sensitive to the cosmological HI signal. In our recent paper, many
signatures from the simulation which predicted this feature were validated
against Murchison Widefield Array (MWA) data but this key pitchfork signature
was close to the noise level. In this paper, we improve the data sensitivity
through coherent averaging of 12 independent snapshots with identical
instrument settings, and provide the first confirmation of the prediction with
a signal-noise ratio > 10. This wide-field effect can be mitigated by careful
antenna designs that suppress sensitivity near the horizon. Simple models for
antenna apertures proposed for future instruments such as the Hydrogen Epoch of
Reionization Array and the Square Kilometre Array indicate they should suppress
foreground leakage from the pitchfork by ~40 dB relative to the MWA, and
significantly increase the likelihood of cosmological signal detection in these
critical Fourier modes in the three-dimensional power spectrum.
Detection of 21 cm emission of HI from the epoch of reionization, at
redshifts z>6, is limited primarily by foreground emission. We investigate the
signatures of wide-field measurements and an all-sky foreground model using the
delay spectrum technique that maps the measurements to foreground object
locations through signal delays between antenna pairs. We demonstrate
interferometric measurements are inherently sensitive to all scales, including
the largest angular scales, owing to the nature of wide-field measurements.
These wide-field effects are generic to all observations but antenna shapes
impact their amplitudes substantially. A dish-shaped antenna yields the most
desirable features from a foreground contamination viewpoint, relative to a
dipole or a phased array. Comparing data from recent Murchison Widefield Array
observations, we demonstrate that the foreground signatures that have the
largest impact on the HI signal arise from power received far away from the
primary field of view. We identify diffuse emission near the horizon as a
significant contributing factor, even on wide antenna spacings that usually
represent structures on small scales. For signals entering through the primary
field of view, compact emission dominates the foreground contamination. These
two mechanisms imprint a characteristic pitchfork signature on the "foreground
wedge" in Fourier delay space. Based on these results, we propose that
selective down-weighting of data based on antenna spacing and time can mitigate
foreground contamination substantially by a factor ~100 with negligible loss of
sensitivity.
The Murchison Widefield Array (MWA) is a new low-frequency interferometric
radio telescope built in Western Australia at one of the locations of the
future Square Kilometre Array (SKA). We describe the automated radio-frequency
interference (RFI) detection strategy implemented for the MWA, which is based
on the AOFlagger platform, and present 72-231-MHz RFI statistics from 10
observing nights. RFI detection removes 1.1% of the data. RFI from digital TV
(DTV) is observed 3% of the time due to occasional ionospheric or atmospheric
propagation. After RFI detection and excision, almost all data can be
calibrated and imaged without further RFI mitigation efforts, including
observations within the FM and DTV bands. The results are compared to a
previously published Low-Frequency Array (LOFAR) RFI survey. The remote
location of the MWA results in a substantially cleaner RFI environment compared
to LOFAR's radio environment, but adequate detection of RFI is still required
before data can be analysed. We include specific recommendations designed to
make the SKA more robust to RFI, including: the availability of sufficient
computing power for RFI detection; accounting for RFI in the receiver design; a
smooth band-pass response; and the capability of RFI detection at high time and
frequency resolution (second and kHz-scale respectively).
The epoch of reionization power spectrum is expected to evolve strongly with
redshift, and it is this variation with cosmic history that will allow us to
begin to place constraints on the physics of reionization. The primary obstacle
to the measurement of the EoR power spectrum is bright foreground emission. We
present an analysis of observations from the Donald C. Backer Precision Array
for Probing the Epoch of Reionization (PAPER) telescope which place new limits
on the HI power spectrum over the redshift range of , extending
previously published single redshift results to cover the full range accessible
to the instrument. To suppress foregrounds, we use filtering techniques that
take advantage of the large instrumental bandwidth to isolate and suppress
foreground leakage into the interesting regions of k-space. Our 500 hour
integration is the longest such yet recorded and demonstrates this method to a
dynamic range of . Power spectra at different points across the redshift
range reveal the variable efficacy of the foreground isolation. Noise limited
measurements of at k=0.2hMpc and z=7.55 reach as low as
(48mK) (). We demonstrate that the size of the error bars in our
power spectrum measurement as generated by a bootstrap method is consistent
with the fluctuations due to thermal noise. Relative to this thermal noise,
most spectra exhibit an excess of power at a few sigma. The likely sources of
this excess include residual foreground leakage, particularly at the highest
redshift, and unflagged RFI. We conclude by discussing data reduction
improvements that promise to remove much of this excess.
The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80-300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ~3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised.
LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer
constructed in the north of the Netherlands and across europe. Utilizing a
novel phased-array design, LOFAR covers the largely unexplored low-frequency
range from 10-240 MHz and provides a number of unique observing capabilities.
Spreading out from a core located near the village of Exloo in the northeast of
the Netherlands, a total of 40 LOFAR stations are nearing completion. A further
five stations have been deployed throughout Germany, and one station has been
built in each of France, Sweden, and the UK. Digital beam-forming techniques
make the LOFAR system agile and allow for rapid repointing of the telescope as
well as the potential for multiple simultaneous observations. With its dense
core array and long interferometric baselines, LOFAR achieves unparalleled
sensitivity and angular resolution in the low-frequency radio regime. The LOFAR
facilities are jointly operated by the International LOFAR Telescope (ILT)
foundation, as an observatory open to the global astronomical community. LOFAR
is one of the first radio observatories to feature automated processing
pipelines to deliver fully calibrated science products to its user community.
LOFAR's new capabilities, techniques and modus operandi make it an important
pathfinder for the Square Kilometre Array (SKA). We give an overview of the
LOFAR instrument, its major hardware and software components, and the core
science objectives that have driven its design. In addition, we present a
selection of new results from the commissioning phase of this new radio
observatory.
Significant new opportunities for astrophysics and cosmology have been
identified at low radio frequencies. The Murchison Widefield Array is the first
telescope in the Southern Hemisphere designed specifically to explore the
low-frequency astronomical sky between 80 and 300 MHz with arcminute angular
resolution and high survey efficiency. The telescope will enable new advances
along four key science themes, including searching for redshifted 21 cm
emission from the epoch of reionisation in the early Universe; Galactic and
extragalactic all-sky southern hemisphere surveys; time-domain astrophysics;
and solar, heliospheric, and ionospheric science and space weather. The
Murchison Widefield Array is located in Western Australia at the site of the
planned Square Kilometre Array (SKA) low-band telescope and is the only
low-frequency SKA precursor facility. In this paper, we review the performance
properties of the Murchison Widefield Array and describe its primary scientific
objectives.
We present the Lyα luminosity function (LF), clustering measurements, and Lyα line profiles based on the largest sample to date of 207 Lyα emitters (LAEs) at z = 6.6 on the 1 deg2 sky of Subaru/XMM-Newton Deep Survey field. Our z = 6.6 Lyα LF including cosmic variance estimates yields the best-fit Schechter parameters of * = 8.5+3.0 –2.2 × 10–4 Mpc–3 and L*Lyα = 4.4+0.6 –0.6 × 1042 erg s–1 with a fixed α = –1.5, and indicates a decrease from z = 5.7 at the 90% confidence level. However, this decrease is not large, only 30% in Lyα luminosity, which is too small to have been identified in the previous studies. A clustering signal of z = 6.6 LAEs is detected for the first time. We obtain the correlation length of r 0= 2-5 h –1 100 Mpc and a bias of b= 3-6, and find no significant boost of clustering amplitude by reionization at z = 6.6. The average hosting dark halo mass inferred from clustering is 1010-1011 M ☉, and a duty cycle of LAE population is roughly ~1%, albeit with large uncertainties. The average of our high-quality Keck/DEIMOS spectra shows an FWHM velocity width of 251 ± 16 km s–1. We find no large evolution of the Lyα line profile from z = 5.7 to 6.6, and no anti-correlation between Lyα luminosity and line width at z = 6.6. The combination of various reionization models and our observational results about the LF, clustering, and line profile indicates that there would exist a small decrease of the intergalactic medium's (IGM's) Lyα transmission owing to reionization, but that the hydrogen IGM is not highly neutral at z = 6.6. Our neutral-hydrogen fraction constraint implies that the major reionization process took place at z 7.
We are developing the Precision Array for Probing the Epoch of Re-ionization (PAPER) to detect 21 cm emission from the early universe, when the first stars and galaxies were forming. We describe the overall experiment strategy and architecture and summarize two PAPER deployments: a four-antenna array in the low radio frequency interference (RFI) environment of Western Australia and an eight-antenna array at a prototyping site at the NRAO facilities near Green Bank, WV. From these activities we report on system performance, including primary beam model verification, dependence of system gain on ambient temperature, measurements of receiver and overall system temperatures, and characterization of the RFI environment at each deployment site. We present an all-sky map synthesized between 139 MHz and 174 MHz using data from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e–5 sr at 156 MHz), with a 10 mJy (620 mK) thermal noise level that indicates what would be achievable with better foreground subtraction. We calculate angular power spectra (C ℓ) in a cold patch and determine them to be dominated by point sources, but with contributions from galactic synchrotron emission at lower radio frequencies and angular wavemodes. Although the sample variance of foregrounds dominates errors in these power spectra, we measure a thermal noise level of 310 mK at ℓ = 100 for a 1.46 MHz band centered at 164.5 MHz. This sensitivity level is approximately 3 orders of magnitude in temperature above the level of the fluctuations in 21 cm emission associated with re-ionization.
Precise subtraction of foreground sources is crucial for detecting and
estimating 21cm HI signals from the Epoch of Reionization (EoR). We quantify
how imperfect point source subtraction due to limitations of the measurement
dataset yields structured residual signal in the dataset. We use the Cramer-Rao
lower bound, as a metric for quantifying the precision with which a parameter
may be measured, to estimate the residual signal in a visibility dataset due to
imperfect point source subtraction. We then propagate these residuals into two
metrics of interest for 21cm EoR experiments - the angular and two-dimensional
power spectrum - using a combination of full analytic covariant derivation,
analytic variant derivation, and covariant Monte Carlo simulations. This
methodology differs from previous work in two ways: (1) it uses information
theory to set the point source position error, rather than assuming a global
root-mean-square error, and (2) it describes a method for propagating the
errors analytically, thereby obtaining the full correlation structure of the
power spectra. The methods are applied to two upcoming low-frequency
instruments: the Murchison Widefield Array and the Precision Array for Probing
the Epoch of Reionization. In addition to the actual antenna configurations, we
apply the methods to minimally-redundant and maximally-redundant
configurations. We find that for peeling sources above 1 Jy, the amplitude of
the residual signal, and its variance, will be smaller than the contribution
from thermal noise for the observing parameters proposed for upcoming EoR
experiments, and that optimal subtraction of bright point sources will not be a
limiting factor for EoR parameter estimation. We then use the formalism to
provide an ab initio analytic derivation motivating the 'wedge' feature in the
two-dimensional power spectrum, complementing previous discussion in the
literature.
Tomography of redshifted 21 cm transition from neutral Hydrogen using Fourier
synthesis telescopes is a promising tool to study the Epoch of Reionization
(EoR). Limiting the confusion from Galactic and Extragalactic foregrounds is
critical to the success of these telescopes. Instrumental response or the Point
Spread Function (PSF) of such telescopes is inherently 3 dimensional with
frequency mapping to the Line of Sight (LOS) distance. EoR signals will
necessarily have to be detected in data where continuum confusion persists;
therefore, it is important that the PSF has acceptable frequency structure so
that the residual foreground does not confuse the EoR signature. This paper
aims to understand the 3 dimensional PSF and foreground contamination in the
same framework. We develop a formalism to estimate the foreground contamination
along frequency, or equivalently LOS dimension, and establish a relationship
between foreground contamination in the image plane and visibility weights on
the Fourier plane. We identify two dominant sources of LOS foreground
contamination-'PSF contamination' and 'gridding contamination'. We show that
'PSF contamination' is localized in LOS wavenumber space, beyond which there
potentially exists an 'EoR window' with negligible foreground contamination
where we may focus our efforts to detect EoR. 'PSF contamination' in this
window may be substantially reduced by judicious choice of a frequency window
function. Gridding and imaging algorithms create additional 'gridding
contamination' and we propose a new imaging algorithm using the Chirp Z
Transform (CZT) that significantly reduces this contamination. Finally, we
demonstrate the analytical relationships and the merit of the new imaging
algorithm for the case of imaging with the Murchison Widefield Array (MWA).
The separation of cosmological signal from astrophysical foregrounds is a fundamental challenge for any effort to probe the evolution of neutral hydrogen during the Cosmic Dawn and epoch of reionization using the 21 cm hyperfine transition. Foreground separation is made possible by their intrinsic spectral smoothness, making them distinguishable from spectrally complex cosmological signal even though they are ∼5 orders of magnitude brighter. Precisely calibrated radio interferometers are essential to maintaining the smoothness and thus separability of the foregrounds. One powerful calibration strategy is to use redundant measurements between pairs of antennas with the same physical separation in order to solve for each antenna’s spectral response without reference to a sky model. This strategy is being employed by the Hydrogen Epoch of Reionization Array (HERA), a large radio telescope in South Africa that is now observing while being built out to 350 14-m dishes. However, the deviations from perfect redundancy inherent in any real radio telescope complicate the calibration problem. Using simulations of HERA, we show how calibration with antenna-to-antenna variations in dish construction and placement generally leads to spectral structure in otherwise smooth foregrounds that significantly reduces the number of cosmological modes available to a 21 cm measurement. However, we also show that this effect can be largely eliminated by a modified redundant-baseline calibration strategy that relies predominantly on short baselines.
The compact configuration of Phase II of the Murchison Widefield Array (MWA) consists of both a redundant subarray and pseudo-random baselines, offering unique opportunities to perform sky-model and redundant interferometric calibration. The highly redundant hexagonal cores give improved power spectrum sensitivity. In this paper, we present the analysis of nearly 40 hr of data targeting one of the MWA’s epoch of reionization (EoR) fields observed in 2016. We use both improved analysis techniques presented in Barry et al. and several additional techniques developed for this work, including data quality control methods and interferometric calibration approaches. We show the EoR power spectrum limits at redshift 6.5, 6.8, and 7.1 based on our deep analysis on this 40 hr data set. These limits span a range in k -space of 0.18 h Mpc ⁻¹ < k < 1.6 h Mpc ⁻¹ , with a lowest measurement of Δ ² ≤ 2.39 × 10 ³ mK ² at k = 0.59 h Mpc ⁻¹ and z = 6.5.
We study the behavior of internal signal chain reflections and antenna cross coupling as systematics for 21 cm cosmological surveys. We outline the mathematics for how these systematics appear in interferometric visibilities and describe their phenomenology. We then describe techniques for modeling and removing these systematics without attenuating the 21 cm signal in the data. This has critical implications for low-frequency radio surveys aiming to characterize the 21 cm signal from the Epoch of Reionization (EoR) and Cosmic Dawn, as systematics can cause bright foreground emission to contaminate the EoR window and prohibit a robust detection. We also quantify the signal loss properties of the systematic modeling algorithms, and show that our techniques demonstrate resistance against EoR signal loss. In a companion paper, we demonstrate these methods on data from the Hydrogen Epoch of Reionization Array as a proof-of-concept.
Context. Experiments that try to observe the 21 cm redshifted signals from the epoch of reionisation (EoR) using interferometric low-frequency instruments have stringent requirements on the processing accuracy.
Aims. We analyse the accuracy of radio interferometric gridding of visibilities with the aim to quantify the power spectrum bias caused by gridding. We do this ultimately to determine the suitability of different imaging algorithms and gridding settings for an analysis of a 21 cm power spectrum.
Methods. We simulated realistic Low-Frequency Array (LOFAR) data and constructed power spectra with convolutional gridding and w stacking, w projection, image-domain gridding, and without w correction. These were compared against data that were directly Fourier transformed. The influence of oversampling, kernel size, w -quantization, kernel windowing function, and image padding were quantified. The gridding excess power was measured with a foreground subtraction strategy, for which foregrounds were subtracted using Gaussian progress regression, as well as with a foreground avoidance strategy.
Results. Constructing a power spectrum with a significantly lower bias than the expected EoR signals is possible with the methods we tested, but requires a kernel oversampling factor of at least 4000, and when w -correction is used, at least 500 w -quantization levels. These values are higher than typically used values for imaging, but they are computationally feasible. The kernel size and padding factor parameters are less crucial. Of the tested methods, image-domain gridding shows the highest accuracy with the lowest imaging time.
Conclusions. LOFAR 21 cm power spectrum results are not affected by gridding. Image-domain gridding is overall the most suitable algorithm for 21 cm EoR power spectrum experiments, including for future analyses of data from the Square Kilometre Array (SKA) EoR. Nevertheless, convolutional gridding with tuned parameters results in sufficient accuracy for interferometric 21 cm EoR experiments. This also holds for w stacking for wide-field imaging. The w -projection algorithm is less suitable because of the requirements for kernel oversampling, and a faceting approach is unsuitable because it causes spatial discontinuities.
We present limits on the 21 cm power spectrum from the Epoch of Reionization using data from the 64 antenna configuration of the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) analyzed through a power spectrum pipeline independent from previous PAPER analyses. Previously reported results from PAPER have been found to contain significant signal loss. Several lossy steps from previous PAPER pipelines have not been included in this analysis, namely delay-based foreground filtering, optimal fringe-rate filtering, and empirical covariance-based estimators. Steps that remain in common with previous analyses include redundant calibration and local sidereal time (LST) binning. The power spectra reported here are effectively the result of applying a linear Fourier transform analysis to the calibrated, LST-binned data. This analysis also uses more data than previous publications, including the complete available redshift range of z ∼ 7.5 to 11. In previous PAPER analyses, many power spectrum measurements were found to be detections of noncosmological power at levels of significance ranging from two to hundreds of times the theoretical noise. Here, excess power is examined using redundancy between baselines and power spectrum jackknives. The upper limits we find on the 21 cm power spectrum from reionization are , , , , , and at redshifts z = 10.87, 9.93, 8.68, 8.37, 8.13, and 7.48, respectively. For reasons described in Cheng et al., these limits supersede all previous PAPER results.
Epoch of Reionisation (EoR) data analysis requires unprecedented levels of accuracy in radio interferometer pipelines. We have developed an imaging power spectrum analysis to meet these requirements and generate robust 21 cm EoR measurements. In this work, we build a signal path framework to mathematically describe each step in the analysis, from data reduction in the Fast Holographic Deconvolution (FHD) package to power spectrum generation in the ε ppsilon package. In particular, we focus on the distinguishing characteristics of FHD/ ε ppsilon: highly accurate spectral calibration, extensive data verification products, and end-to-end error propagation. We present our key data analysis products in detail to facilitate understanding of the prominent systematics in image-based power spectrum analyses. As a verification to our analysis, we also highlight a full-pipeline analysis simulation to demonstrate signal preservation and lack of signal loss. This careful treatment ensures that the FHD/ ε ppsilon power spectrum pipeline can reduce radio interferometric data to produce credible 21 cm EoR measurements.
This is the fourth paper in a series of publications aiming at discovering quasars at the epoch of reionization. In this paper, we expand our search for z ∼ 7 quasars to the footprint of the Dark Energy Survey (DES) Data Release One (DR1), covering ∼5000 deg ² of a new area. We select z ∼ 7 quasar candidates using deep optical, near-infrared (near-IR) and mid-infrared (mid-IR) photometric data from the DES DR1, the VISTA Hemisphere Survey, the VISTA Kilo-degree Infrared Galaxy survey, the UKIRT InfraRed Deep Sky Surveys—Large Area Survey (ULAS), and the unblurred coadds from the Wide-field Infrared Survey Explore ( WISE ) images (unWISE). The inclusion of DES and unWISE photometry allows the search to reach ∼1 mag fainter, comparing to our z ≳ 6.5 quasar survey in the northern sky. We report the initial discovery and spectroscopic confirmation of six new luminous quasars at z > 6.4, including an object at z = 7.02, the fourth quasar yet known at z > 7, from a small fraction of candidates observed thus far. Based on the recent measurement of z ∼ 6.7 quasar luminosity function using the quasar sample from our survey in the northern sky, we estimate that there will be ≳55 quasars at z > 6.5 at M 1450 < −24.5 in the full DES footprint.
The advent of a new generation of low-frequency interferometers has opened a direct window into the Epoch of Reionization (EoR). However, key to a detection of the faint 21 cm signal, and reaching the sensitivity limits of these arrays, is a detailed understanding of the instruments and their calibration. In this work, we use simulations to investigate the bias and uncertainty of redundancy-based calibration. Specifically, we study the influence of the flux distribution of the radio sky and the impact of antenna position offsets on the complex calibration solutions. We find that the position offsets introduce a bias into phase component of the calibration solutions. This phase bias increases with the distance between bright radio sources and the pointing center, and with the flux density of these sources. This is potentially problematic for redundant calibration on Murchison Widefield Array (MWA) observations of EoR fields 1 and 2. EoR field 0, however, lacks such sources. We also compared the simulations with theoretical estimates for the bias and uncertainty in sky-model-based calibration on incomplete sky models for the redundant antenna tiles in the MWA. Our results indicate that redundant calibration outperforms sky-based calibration due to the high positional precision of the MWA antenna tiles.
We describe the motivation and design details of the ‘Phase II’ upgrade of the Murchison Widefield Array radio telescope. The expansion doubles to 256 the number of antenna tiles deployed in the array. The new antenna tiles enhance the capabilities of the Murchison Widefield Array in several key science areas. Seventy-two of the new tiles are deployed in a regular configuration near the existing array core. These new tiles enhance the surface brightness sensitivity of the array and will improve the ability of the Murchison Widefield Array to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ∼3.5. The remaining 56 tiles are deployed on long baselines, doubling the maximum baseline of the array and improving the array u, v coverage. The improved imaging capabilities will provide an order of magnitude improvement in the noise floor of Murchison Widefield Array continuum images. The upgrade retains all of the features that have underpinned the Murchison Widefield Array’s success (large field of view, snapshot image quality, and pointing agility) and boosts the scientific potential with enhanced imaging capabilities and by enabling new calibration strategies.
The Epoch of Reionization (EoR) is an uncharted era in our universe's history during which the birth of the first stars and galaxies led to the ionization of neutral hydrogen in the intergalactic medium. There are many experiments investigating the EoR by tracing the 21 cm line of neutral hydrogen. Because this signal is very faint and difficult to isolate, it is crucial to develop analysis techniques that maximize sensitivity and suppress contaminants in data. It is also imperative to understand the trade-offs between different analysis methods and their effects on power spectrum estimates. Specifically, with a statistical power spectrum detection in HERA's foreseeable future, it has become increasingly important to understand how certain analysis choices can lead to the loss of the EoR signal. In this paper, we focus on signal loss associated with power spectrum estimation. We describe the origin of this loss using both toy models and data taken by the 64-element configuration of the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER). In particular, we highlight how detailed investigations of signal loss have led to a revised, higher 21 cm power spectrum upper limit from PAPER-64. Additionally, we summarize errors associated with power spectrum error estimation that were previously unaccounted for. We focus on a subset of PAPER-64 data in this paper; revised power spectrum limits from the PAPER experiment are presented in a forthcoming paper by Kolopanis et al. and supersede results from previously published PAPER analyses. © 2018. The American Astronomical Society. All rights reserved.
Interferometric arrays seeking to measure the 21 cm signal from the epoch of reionization (EOR) must contend with overwhelmingly bright emission from foreground sources. Accurate recovery of the 21 cm signal will require precise calibration of the array, and several new avenues for calibration have been pursued in recent years, including methods using redundancy in the antenna configuration. The newly upgraded Phase II of Murchison Widefield Array (MWA) is the first interferometer that has large numbers of redundant baselines while retaining good instantaneous UV coverage. This array therefore provides a unique opportunity to compare redundant calibration with sky-model-based algorithms. In this paper, we present the first results from comparing both calibration approaches with MWA Phase II observations. For redundant calibration, we use the package OMNICAL and produce sky-based calibration solutions with the analysis package Fast Holographic Deconvolution (FHD). There are three principal results: (1) We report the success of OMNICAL on observations of ORBComm satellites, showing substantial agreement between redundant visibility measurements after calibration. (2) We directly compare OMNICAL calibration solutions with those from FHD and demonstrate that these two different calibration schemes give extremely similar results. (3) We explore improved calibration by combining OMNICAL and FHD. We evaluate these combined methods using power spectrum techniques developed for EOR analysis and find evidence for marginal improvements mitigating artifacts in the power spectrum. These results are likely limited by the signal-to-noise ratio in the 6 hr of data used, but they suggest future directions for combining these two calibration schemes.
The Murchison Widefield Array (MWA), located in Western Australia, is one of the low-frequency precursors of the international Square Kilometre Array (SKA) project. In addition to pursuing its own ambitious science program, it is also a testbed for wide range of future SKA activities ranging from hardware, software to data analysis. The key science programs for the MWA and SKA require very high dynamic ranges, which challenges calibration and imaging systems. Correct calibration of the instrument and accurate measurements of source flux densities and polarisations require precise characterisation of the telescope's primary beam. Recent results from the MWA GaLactic Extragalactic All-sky MWA (GLEAM) survey show that the previously implemented Average Embedded Element (AEE) model still leaves residual polarisations errors of up to 10-20 % in Stokes Q. We present a new simulation-based Full Embedded Element (FEE) model which is the most rigorous realisation yet of the MWA's primary beam model. It enables efficient calculation of the MWA beam response in arbitrary directions without necessity of spatial interpolation. In the new model, every dipole in the MWA tile (4 x 4 bow-tie dipoles) is simulated separately, taking into account all mutual coupling, ground screen and soil effects, and therefore accounts for the different properties of the individual dipoles within a tile. We have applied the FEE beam model to GLEAM observations at 200 - 231 MHz and used false Stokes parameter leakage as a metric to compare the models. We have determined that the FEE model reduced the magnitude and declination-dependent behaviour of false polarisation in Stokes Q and V while retaining low levels of false polarisation in Stokes U.
The current generation of experiments aiming to detect the neutral hydrogen signal from the Epoch of Reionisation (EoR) is likely to be limited by systematic effects associated with removing foreground sources from target fields. In this paper we develop a model for the compact foreground sources in one of the target fields of the MWA's EoR key science experiment: the `EoR1' field. The model is based on both the MWA's GLEAM survey and GMRT 150 MHz data from the TGSS survey, the latter providing higher angular resolution and better astrometric accuracy for compact sources than is available from the MWA alone. The model contains 5049 sources, some of which have complicated morphology in MWA data, Fornax A being the most complex. The higher resolution data show that 13% of sources that appear point-like to the MWA have complicated morphology such as double and quad structure, with a typical separation of 33~arcsec. We derive an analytic expression for the error introduced into the EoR two-dimensional power spectrum due to peeling close double sources as single point sources and show that for the measured source properties, the error in the power spectrum is confined to high modes that do not affect the overall result for the large-scale cosmological signal of interest. The brightest ten mis-modelled sources in the field contribute 90% of the power bias in the data, suggesting that it is most critical to improve the models of the brightest sources. With this hybrid model we reprocess data from the EoR1 field and show a maximum of 8% improved calibration accuracy and a factor of two reduction in residual power in k-space from peeling these sources. Implications for future EoR experiments including the SKA are discussed in relation to the improvements obtained.
We test the bandpass smoothness performance of two prototype Square Kilometre Array (SKA) SKA1-Low log-periodic dipole antennas, the SKALA2 and SKALA3 (`SKA Log-periodic Antenna'), and the current dipole from the Murchison Widefield Array (MWA) precursor telescope. Throughout this paper, we refer to the output complex-valued voltage response of an antenna when connected to a low noise amplifier (LNA), as the dipole bandpass. In Paper I (de Lera Acedo et al. 2017), the bandpass spectral response of the log-periodic antenna being developed for the SKA1-Low was estimated using numerical electromagnetic simulations and analyzed using low-order polynomial fittings and it was compared with the HERA antenna against the delay spectrum metric. In this work, realistic simulations of the SKA1-Low instrument, including frequency-dependent primary beams and array configuration, are used with a weighted least-squares polynomial estimator to assess the ability of prototype antennas to perform the SKA Epoch of Reionisation (EoR) statistical experiments. This work complements the ideal estimator tolerances computed for the proposed EoR science experiments in Trott & Wayth (2016), with the realised performance of an optimal and standard estimation (calibration) procedure. With a sufficient sky calibration model at higher frequencies, all antennas have bandpasses that are sufficiently smooth to meet the tolerances described in Trott & Wayth (2016) to perform the EoR statistical experiments, and these are primarily limited by an adequate sky calibration model, and the thermal noise level in the calibration data. At frequencies of the Cosmic Dawn (CD), which is of principal interest to SKA as one of the first next-generation telescopes capable of accessing higher redshifts, the MWA dipole and SKALA3 antenna have adequate performance, while the SKALA2 design will impede the ability to explore this era.
We present the first limits on the Epoch of Reionization (EoR) 21-cm HI power spectra, in the redshift range , using the Low-Frequency Array (LOFAR) High-Band Antenna (HBA). In total 13\,h of data were used from observations centred on the North Celestial Pole (NCP). After subtraction of the sky model and the noise bias, we detect a non-zero (1-) excess variance and a best 2- upper limit of at k=0.053hcMpc in the range z=9.6-10.6. The excess variance decreases when optimizing the smoothness of the direction- and frequency-dependent gain calibration, and with increasing the completeness of the sky model. It is likely caused by (i) residual side-lobe noise on calibration baselines, (ii) leverage due to non-linear effects, (iii) noise and ionosphere-induced gain errors, or a combination thereof. Further analyses of the excess variance will be discussed in forthcoming publications.
This paper is the first on a series of papers describing the impact of antenna instrumental artefacts on the 21-cm cosmology experiments to be carried out by the SKA1-LOW telescope, i.e., the Cosmic Dawn (CD) and the Epoch of Reionization (EoR). The smoothness of the passband response of the current log-periodic antenna being developed for the SKA1-LOW is analyzed using numerical electromagnetic simulations. The frequency ripples are characterized using low-order polynomials defined locally, in order to study the impact of the passband smoothness in the instrument calibration and CD/EoR Science. A solution is offered to correct a fast ripple found at 60 MHz during a test campaign at the SKA site at the Murchison Radio-astronomy Observatory, Western Australia in September 2015 with a minor impact on the telescope's performance and design. A comparison with the Hydrogen Epoch of Reionization Array antenna is also shown demonstrating the potential use of the SKA1-LOW antenna for the Delay Spectrum technique to detect the EoR.
We study the impact of sky-based calibration errors from source mismodeling on 21 cm power spectrum measurements with an interferometer and propose a method for suppressing their effects. While emission from faint sources that are not accounted for in calibration catalogs is believed to be spectrally smooth, deviations of true visibilities from model visibilities are not, due to the inherent chromaticity of the interferometer's sky-response (the "wedge"). Thus, unmodeled foregrounds at the mJy level introduce frequency structure into gain solutions on the same line-of-sight scales on which we hope to observe the cosmological signal. We derive analytic expressions describing these errors using linearized approximations of the calibration equations and determine the impact of this bias on measurements of the 21 cm power spectrum during the Epoch of Reionization (EoR). Given our current precision in primary beam and foreground modeling, this noise will significantly impact the sensitivity of existing experiments that rely on sky-based calibration rather than redundant calibration. Sky-based calibration that down-weights long baselines can eliminate contamination in most of the region outside of the wedge with only a modest increase in instrumental noise.
The Murchison Widefield Array (MWA) has collected hundreds of hours of Epoch of Reionization (EoR) data and now faces the challenge of overcoming foreground and systematic contamination to reduce the data to a cosmological measurement. We introduce several novel analysis techniques such as cable reflection calibration, hyper-resolution gridding kernels, diffuse foreground model subtraction, and quality control methods. Each change to the analysis pipeline is tested against a two dimensional power spectrum figure of merit to demonstrate improvement. We incorporate the new techniques into a deep integration of 32 hours of MWA data. This data set is used to place a systematic-limited upper limit on the cosmological power spectrum of mK at k=0.27 h~Mpc and z=7.1, consistent with other published limits, and a modest improvement (factor of 1.4) over previous MWA results. From this deep analysis we have identified a list of improvements to be made to our EoR data analysis strategies. These improvements will be implemented in the future and detailed in upcoming publications.
LOFAR is the LOw Frequency Radio interferometer ARray located at mid-latitude (). Here, we present results on ionospheric structures derived from 29 LOFAR nighttime observations during the winters of 2012/2013 and 2013/2014. We show that LOFAR is able to determine differential ionospheric TEC values with an accuracy better than 1 mTECU over distances ranging between 1 and 100 km. For all observations the power law behavior of the phase structure function is confirmed over a long range of baseline lengths, between 1 and 80 km, with a slope that is in general larger than the 5/3 expected for pure Kolmogorov turbulence. The measured average slope is 1.89 with a one standard deviation spread of 0.1. The diffractive scale, i.e. the length scale where the phase variance is , is shown to be an easily obtained single number that represents the ionospheric quality of a radio interferometric observation. A small diffractive scale is equivalent to high phase variability over the field of view as well as a short time coherence of the signal, which limits calibration and imaging quality. For the studied observations the diffractive scales at 150 MHz vary between 3.5 and km. A diffractive scale above 5 km, pertinent to about of the observations, is considered sufficient for the high dynamic range imaging needed for the LOFAR Epoch of Reionization project. For most nights the ionospheric irregularities were anisotropic, with the structures being aligned with the Earth magnetic field in about of the observations.
We present the 21 cm power spectrum analysis approach of the Murchison Widefield Array Epoch of Reionization project. In this paper, we compare the outputs of multiple pipelines for the purpose of validating statistical limits cosmological hydrogen at redshifts between 6 and 12. Multiple, independent, data calibration and reduction pipelines are used to make power spectrum limits on a fiducial night of data. Comparing the outputs of imaging and power spectrum stages highlights differences in calibration, foreground subtraction and power spectrum calculation. The power spectra found using these different methods span a space defined by the various tradeoffs between speed, accuracy, and systematic control. Lessons learned from comparing the pipelines range from the algorithmic to the prosaically mundane; all demonstrate the many pitfalls of neglecting reproducibility. We briefly discuss the way these different methods attempt to handle the question of evaluating a significant detection in the presence of foregrounds.
Spectral features introduced by instrumental chromaticity of radio interferometers have the potential to negatively impact the ability to perform Epoch of Reionisation and Cosmic Dawn (EoR/CD) science. We describe instrument calibration choices that influence the spectral characteristics of the science data, and assess their impact on EoR/CD statistical and tomographic experiments. Principally, we consider the intrinsic spectral response of the antennas, embedded within a complete frequency-dependent primary beam response, and instrument sampling. The analysis is applied to the proposed SKA1-Low EoR/CD experiments. We provide tolerances on the smoothness of the SKA station primary beam bandpass, to meet the scientific goals of statistical and tomographic (imaging) of EoR/CD programs. Two calibration strategies are tested: (1) fitting of each fine channel independently, and (2) fitting of an
n
th-order polynomial for each ~ 1 MHz coarse channel with (
n
+1)th-order residuals (
n
= 2, 3, 4). Strategy (1) leads to uncorrelated power in the 2D power spectrum proportional to the thermal noise power, thereby reducing the overall sensitivity. Strategy (2) leads to correlated residuals from the fitting, and residual signal power with (
n
+1)th-order curvature. For the residual power to be less than the thermal noise, the fractional amplitude of a fourth-order term in the bandpass across a single coarse channel must be < 2.5% (50 MHz), < 0.5% (150 MHz), < 0.8% (200 MHz). The tomographic experiment places constraints on phase residuals in the bandpass. We find that the root-mean-square variability over all stations of the change in phase across any fine channel (4.578 kHz) should not exceed 0.2 degrees.
21 cm Epoch of Reionization observations promise to transform our understanding of galaxy formation, but these observations
are impossible without unprecedented levels of instrument calibration. We present end-to-end simulations of a full EoR power
spectrum analysis including all of the major components of a real data processing pipeline: models of astrophysical foregrounds
and EoR signal, frequency-dependent instrument effects, sky-based antenna calibration, and the full PS analysis. This study
reveals that traditional sky-based per-frequency antenna calibration can only be implemented in EoR measurement analyses if
the calibration model is unrealistically accurate. For reasonable levels of catalogue completeness, the calibration introduces
contamination in otherwise foreground-free power spectrum modes, precluding a PS measurement. We explore the origin of this
contamination and potential mitigation techniques. We show that there is a strong joint constraint on the precision of the
calibration catalogue and the inherent spectral smoothness of antennae, and that this has significant implications for the
instrumental design of the SKA and other future EoR observatories.
Detection of the cosmological neutral hydrogen signal from the Epoch of
Reionization, and estimation of its basic physical parameters, is the principal
scientific aim of many current low-frequency radio telescopes. Here we describe
the Cosmological HI Power Spectrum Estimator (CHIPS), an algorithm developed
and implemented with data from the Murchison Widefield Array (MWA), to compute
the two-dimensional and spherically-averaged power spectrum of brightness
temperature fluctuations. The principal motivations for CHIPS are the
application of realistic instrumental and foreground models to form the optimal
estimator, thereby maximising the likelihood of unbiased signal estimation, and
allowing a full covariant understanding of the outputs. CHIPS employs an
inverse-covariance weighting of the data through the maximum likelihood
estimator, thereby allowing use of the full parameter space for signal
estimation ("foreground suppression"). We describe the motivation for the
algorithm, implementation, application to real and simulated data, and early
outputs. Upon application to a set of 3 hours of data, we set a 2 upper
limit on the EoR dimensionless power at k=0.05~h.Mpc of
~mK in the redshift range ,
consistent with previous estimates.
The Long Wavelength Array (LWA) will be a new, open, user-oriented astronomical instrument operating in the relatively unexplored window from 20–80 MHz near arcsecond angular resolution and milliJansky sensitivity. Operated by the University of New Mexico on behalf of the Southwest Consortium (SWC) the LWA will provide a unique training ground for the next generation of radio astronomers. Students may also put skills learned on the LWA to work in computer science, electrical engineering, and the communications industry, among others. The development of the LWA will follow a phased build which benefits from lessons learned at each phase. Four university-based Scientific Testing and Evaluation (ST&E) teams with different areas of concentration: (i) high-resolution imaging and particle acceleration; (ii) wide-field imaging and large scale structures; (iii) ionospheric physics; and (iv) radio frequency interference (RFI) suppression and transient detection will provide the feedback needed to assure that science objectives are met as the build develops. Currently in its first year of construction funding, the LWA team is working on the design for the first station (see also Ray et al. 2006).
Geostatistics offers a variety of models, methods and techniques for the analysis, estimation and display of multivariate data distributed in space or time. The book presents a brief review of statistical concepts, a detailed introduction to linear geostatistics, and an account of three basic methods of multivariate analysis. It contains an advanced presentation of linear models for multivariate spatial or temporal data, including the bilinear model of coregionalization, and an introduction to non-stationary geostatistics with a special focus on the external drift method. The 30 chapters are presented in five parts: preliminaries, geostatistics, multivariate analysis, multivariate geostatistics, non-stationary geostatistics. -from Publisher
We study the evolution of the ionization state of the intergalactic medium (IGM) at the end of the reionization epoch using moderate-resolution spectra of a sample of 19 quasars at 5.74 < zem < 6.42 discovered in the Sloan Digital Sky Survey. Three methods are used to trace IGM properties: (1) the evolution of the Gunn-Peterson (GP) optical depth in the Lyα, Lyβ, and Lyγ transitions; (2) the distribution of lengths of dark absorption gaps; and (3) the size of H II regions around luminous quasars. Using this large sample, we find that the evolution of the ionization state of the IGM accelerated at z > 5.7: the GP optical depth evolution changes from τ ~ (1 + z)4.3 to (1 + z)11, and the average length of dark gaps with τ > 3.5 increases from <10 to >80 comoving Mpc. The dispersion of IGM properties along different lines of sight also increases rapidly, implying fluctuations by a factor of 4 in the UV background at z > 6, when the mean free path of UV photons is comparable to the correlation length of the star-forming galaxies that are thought to have caused reionization. The mean length of dark gaps shows the most dramatic increase at z ~ 6, as well as the largest line-of-sight variations. We suggest using dark gap statistics as a powerful probe of the ionization state of the IGM at yet higher redshift. The sizes of H II regions around luminous quasars decrease rapidly toward higher redshift, suggesting that the neutral fraction of the IGM has increased by a factor of 10 from z = 5.7 to 6.4, consistent with the value derived from the GP optical depth. The mass-averaged neutral fraction is 1%-4% at z ~ 6.2 based on the GP optical depth and H II region size measurements. The observations suggest that z ~ 6 is the end of the overlapping stage of reionization and are inconsistent with a mostly neutral IGM at z ~ 6, as indicated by the finite length of the dark absorption gaps.
We introduce a powerful seminumeric modelling tool, 21cmfast, designed to efficiently simulate the cosmological 21-cm signal. Our code generates 3D realizations of evolved density, ionization,
peculiar velocity and spin temperature fields, which it then combines to compute the 21-cm brightness temperature. Although
the physical processes are treated with approximate methods, we compare our results to a state-of-the-art large-scale hydrodynamic
simulation, and find good agreement on scales pertinent to the upcoming observations (≳1 Mpc). The power spectra from 21cmfast agree with those generated from the numerical simulation to within 10s of per cent, down to the Nyquist frequency. We show
results from a 1-Gpc simulation which tracks the cosmic 21-cm signal down from z= 250, highlighting the various interesting epochs. Depending on the desired resolution, 21cmfast can compute a redshift realization on a single processor in just a few minutes. Our code is fast, efficient, customizable
and publicly available, making it a useful tool for 21-cm parameter studies.
The interferometric technique known as peeling addresses many of the challenges faced when observing with low-frequency radio arrays, and is a promising tool for the associated calibration systems. We investigate a real-time peeling implementation for next-generation radio interferometers such as the Murchison widefield array (MWA). The MWA is being built in Australia and will observe the radio sky between 80 and 300 MHz. The data rate produced by the correlator is just over 19 GB/s (a few peta-bytes/day). It is impractical to store data generated at this rate, and software is currently being developed to calibrate and form images in real time. The software will run on-site on a high-throughput real-time computing cluster at several tera-flops, and a complete cycle of calibration and imaging will be completed every 8 s. Various properties of the implementation are investigated using simulated data. The algorithm is seen to work in the presence of strong galactic emission and with various ionospheric conditions. It is also shown to scale well as the number of antennas increases, which is essential for many upcoming instruments. Lessons from MWA pipeline development and processing of simulated data may be applied to future low-frequency fixed dipole arrays.