Publications (41)9.8 Total impact
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Article: The HI column density distribution function in faint dwarf galaxies
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ABSTRACT: We present the HI column density distribution function,\fnh, as measured from dwarf galaxies observed as part of the Faint Irregular Galaxy GMRT (FIGGS) survey. We find that the shape of the dwarf galaxy \fnh\ is significantly different from the \fnh\ for high redshift Damped \lya\ absorbers (DLAs) or the \fnh\ for a representative sample of $z = 0$ gas rich galaxies. The dwarf \fnh\ falls much more steeply at high HI column densities as compared to the other determinations. While $\sim 10%$ of the cross section above $\nh = 10^{20.3} \acc$ at $z = 0$ is provided by dwarf galaxies, the fraction falls to $\lesssim 1%$ by $\nh \sim 10^{21.5} \acc.$ In the local universe, the contribution to the high \nh\ end of the \fnh\ distribution comes predominantly from the inclined disks of large galaxies. Dwarf galaxies, both because of their smaller scale lengths, and their larger intrinsic axial ratios do not produce large HI column densities even when viewed edge-on. If high column density DLAs/GRB hosts correspond to galaxies like the local dwarfs, this would require either that (i) the absorption arises from merging and not isolated systems or (ii) the observed lines of sight are strongly biased towards high column density regions.11/2012; -
Article: Probing interstellar turbulence in spiral galaxies using HI power spectrum analysis
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ABSTRACT: We estimate the \HI intensity fluctuation power spectrum for a sample of 18 spiral galaxies chosen from THINGS. Our analysis spans a large range of length-scales from $\sim 300 {\rm pc}$ to $\sim 16 {\rm kpc}$ across the entire galaxy sample. We find that the power spectrum of each galaxy can be well fitted by a power law $P_{\rm HI}(U) = A U^{\alpha}$, with an index $\alpha$ that varies from galaxy to galaxy. For some of the galaxies the scale-invariant power-law power spectrum extends to length-scales that are comparable to the size of the galaxy's disk. The distribution of $\alpha$ is strongly peaked with 50% of the values in the range $\alpha=-1.9$ to 1.5, and a mean and standard deviation of -1.3 and 0.5 respectively. We find no significant correlation between $\alpha$ and the star formation rate, dynamical mass, \HI mass or velocity dispersion of the galaxies. Several earlier studies that have measured the power spectrum within our Galaxy on length-scales that are considerably smaller than $500 {\rm pc}$ have found a power-law power spectrum with $\alpha$ in the range $\approx -2.8$ to -2.5. We propose a picture where we interpret the values in the range $\approx -2.8$ to -2.5 as arising from three dimensional (3D) turbulence in the Interstellar Medium (ISM) on length-scales smaller than the galaxy's scale-height, and we interpret the values in the range $\approx -1.9$ to -1.5 measured in this paper as arising from two-dimensional ISM turbulence in the plane of the galaxy's disk. It however still remains a difficulty to explain the small galaxy to galaxy variations in the values of $\alpha$ measured here.08/2012; -
Article: Characterizing Foreground for redshifted 21-cm radiation: 150 MHz GMRT observations
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ABSTRACT: Foreground removal is a major challenge for detecting the redshifted 21-cm neutral hydrogen (HI) signal from the Epoch of Reionization (EoR). We have used 150 MHz GMRT observations to characterize the statistical properties of the foregrounds in four different fields of view. The measured multi-frequency angular power spectrum C_l(Delta nu) is found to have values in the range 10^4 mK^2 to 2 x 10^4 mK^2 across 700 <= l <= 2 x 10^4 and Delta nu <= 2.5 MHz, which is consistent with model predictions where point sources are the most dominant foreground component. The measured C_l(Delta nu) does not show a smooth Delta nu dependence, which poses a severe difficulty for foreground removal using polynomial fitting. The observational data was used to assess point source subtraction. Considering the brightest source (~ 1 Jy) in each field, we find that the residual artifacts are less than 1.5% in the most sensitive field (FIELD I). We have used FIELD I, which has a rms noise of 1.3 mJy/Beam, to study the properties of the radio source population to a limiting flux of 9 mJy. The differential source count is well fitted with a single power law of slope -1.6. We find there is no evidence for flattening of the source counts towards lower flux densities which suggests that source population is dominated by the classical radio-loud Active Galactic Nucleus (AGN). The diffuse Galactic emission is revealed after the point sources are subtracted out from FIELD I . We find C_l \propto l^{-2.34} for 253 <= l <= 800 which is characteristic of the Galactic synchrotron radiation measured at higher frequencies and larger angular scales. We estimate the fluctuations in the Galactic synchrotron emission to be sqrt{l(l+1)C_l/2 pi} ~ 10 K at l=800 (theta > 10'). The measured C_l is dominated by the residual point sources and artifacts at smaller angular scales where C_l ~ 10^3 mK^2 for l > 800.08/2012; -
Article: Atomic hydrogen, star formation and feedback in the lowest mass Blue Compact Dwarf galaxies
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ABSTRACT: We present the results from a search for HI emission from a sample of newly discovered dwarf galaxies in the M81 group. HI is detected in three galaxies, all of which are classified as BCDs. The HI masses of these galaxies are ~ 10^6 M_sun, making these some of the lowest mass BCDs known. For these three galaxies FUV images (from GALEX) and H-alpha images (from the Russian 6m BTA telescope) are available.The H-alpha emission is very faint, and, in principle could be produced by a single O star. Further, in all cases we find offsets between the peak of the FUV emission and that of the H-alpha emission. Offsets between the most recent sites of star formation (i.e. those traced by H-alpha) and the older sites (i.e. those traced by FUV) would be natural if the star formation is stochastic. In spite of the expectation that the effects of mechanical feedback from star formation would be most directly seen in the smallest galaxies with low gravitational potentials, we only see tentative evidence of outflowing HI gas associated with the star forming region in one of the galaxies.07/2012; -
Article: The radio - far infrared correlation in the faintest star forming dwarf galaxies
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ABSTRACT: We study the radio - far-infrared (FIR) correlation in a sample of faint dwarf irregular galaxies using NVSS data for 1.4 GHz radio flux, Spitzer MIPS 70 um data for FIR flux, and GALEX FUV data to estimate the star formation rates (SFR). Since our target galaxies are extremely faint, we stack images of many galaxies together to estimate the average radio and FIR fluxes. We find that for a given SFR both 70 um and 1.4 GHz fluxes are low compared to the calibration for large spirals. Nonetheless, the ratio of 70 um to 1.4 GHz flux agrees within errorbars with that seen for large galaxies. The radio-FIR correlation thus appears to be the result of a 'conspiracy'. We use the SFR to estimate the non-thermal fraction of the 1.4 GHz radio emission and find it to be around 50%, much smaller than the 90% typical for spirals. We also estimate the equipartition magnetic field and find it to be ~ 2 microgauss, about five times smaller than that typical for spirals.04/2012; -
Article: Improved foreground removal in GMRT 610 MHz observations towards redshifted 21-cm tomography
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ABSTRACT: Foreground removal is a challenge for 21-cm tomography of the high redshift Universe. We use archival GMRT data (obtained for completely different astronomical goals) to estimate the foregrounds at a redshift ~ 1. The statistic we use is the cross power spectrum between two frequencies separated by \Delta{\nu} at the angular multipole l, or equivalently the multi-frequency angular power spectrum C_l(\Delta{\nu}). An earlier measurement of C_l(\Delta{\nu}) using this data had revealed the presence of oscillatory patterns along \Delta{\nu}, which turned out to be a severe impediment for foreground removal (Ghosh et al. 2011). Using the same data, in this paper we show that it is possible to considerably reduce these oscillations by suppressing the sidelobe response of the primary antenna elements. The suppression works best at the angular multipoles l for which there is a dense sampling of the u-v plane. For three angular multipoles l = 1405, 1602 and 1876, this sidelobe suppression along with a low order polynomial fitting completely results in residuals of (\leq 0.02 mK^2), consistent with the noise at the 3{\sigma} level. Since the polynomial fitting is done after estimation of the power spectrum it can be ensured that the estimation of the HI signal is not biased. The corresponding 99% upper limit on the HI signal is xHI b \leq 2.9, where xHI is the mean neutral fraction and b is the bias.08/2011; -
Article: Small Bites: Star formation recipes in extreme dwarfs
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ABSTRACT: We study the relationship between the gas column density (Sigma_HI) and the star formation rate surface density (Sigma_SFR) for a sample of extremely small (M_B ~ -13, Delta V_50 ~ 30 km/s) dwarf irregular galaxies. We find a clear stochasticity in the relation between the gas column density and star formation. All gas with Sigma_HI >~ 10 M_sun/pc^2 has some ongoing star formation, but the fraction of gas with ongoing star formation decreases as the gas column density decreases, and falls to about 50% at Sigma_HI ~ 3 M_sun/pc^2. Further, even for the most dense gas, the star formation efficiency is at least a factor of ~ 2 smaller than typical of star forming regions in spirals. We also find that the ratio of H-alpha emission to FUV emission increases with increasing gas column density. This is unlikely to be due to increasing dust extinction because the required dust to gas ratios are too high. We suggest instead that this correlation arises because massive (i.e. H-alpha producing) stars are formed preferentially in regions with high gas density.03/2011; -
Article: GMRT observation towards detecting the Post-reionization 21-cm signal
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ABSTRACT: We have analyzed 610 MHz GMRT observations towards detecting the redshifted 21-cm signal from z=1.32. The multi-frequency angular power spectrum C_l(Delta nu) is used to characterize the statistical properties of the background radiation across angular scales ~20" to 10', and a frequency bandwidth of 7.5 MHz with resolution 125 kHz. The measured C_l(Delta nu) which ranges from 7 mK^2 to 18 mK^2 is dominated by foregrounds, the expected HI signal C_l^HI(Delta nu) ~10^{-6}- 10^{-7} mK^2 is several orders of magnitude smaller. The foregrounds, believed to originate from continuum sources, is expected to vary smoothly with Delta nu whereas the HI signal decorrelates within ~0.5 MHz and this holds the promise of separating the two. For each l, we use the interval 0.5 < Delta nu < 7.5 MHz to fit a fourth order polynomial which is subtracted from the measured C_l(Delta nu) to remove any smoothly varying component across the entire bandwidth Delta nu < 7.5 MHz. The residual C_l(Delta nu), we find, has an oscillatory pattern with amplitude and period respectively ~0.1 mK^2 and Delta nu = 3 MHz at the smallest l value of 1476, and the amplitude and period decreasing with increasing l. Applying a suitably chosen high pass filter, we are able to remove the residual oscillatory pattern for l=1476 where the residual C_l(Delta nu) is now consistent with zero at the 3-sigma noise level. We conclude that we have successfully removed the foregrounds at l=1476 and the residuals are consistent with noise. We use this to place an upper limit on the HI signal whose amplitude is determined by x_HI b where x_HI and b are the HI neutral fraction and the HI bias respectively. A value of x_HI b greater than 7.95 would have been detected in our observation, and is therefore ruled out at the 3-sigma level. (abridged)10/2010; -
Article: Turbulence in the Harassed Galaxy NGC 4254
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ABSTRACT: Galaxy harassment is an important mechanism for the morphological evolution of galaxies in clusters. The spiral galaxy NGC 4254 in the Virgo cluster is believed to be a harassed galaxy. We have analyzed the power spectrum of HI emission fluctuations from NGC 4254 to investigate whether it carries any imprint of galaxy harassment. The power spectrum, as determined using the 16 central channels which contain most of the HI emission, is found to be well fitted by a power law $P(U)=AU^{\alpha}$ with $\alpha\ =-\ 1.7\pm 0.2$ at length-scales $1.7 \, {\rm k pc}$ to $ 8.4 \, {\rm kpc}$. This is similar to other normal spiral galaxies which have a slope of $\sim -1.5$ and is interpreted as arising from two dimensional turbulence at length-scales larger than the galaxy's scale-height. NGC 4254 is hence yet another example of a spiral galaxy that exhibits scale-invariant density fluctuations out to length-scales comparable to the diameter of the HI disk. While a large variety of possible energy sources like proto-stellar winds, supernovae, shocks, etc. have been proposed to produce turbulence, it is still to be seen whether these are effective on length-scales comparable to that of the entire HI disk. On separately analyzing the HI power spectrum in different parts of NGC 4254, we find that the outer parts have a different slope ($ \alpha = -2.0\pm0.3$) compared to the central part of the galaxy ($\alpha = -1.5\pm0.2$). Such a change in slope is not seen in other, undisturbed galaxies. We suggest that, in addition to changing the overall morphology, galaxy harassment also effects the fine scale structure of the ISM, causing the power spectrum to have a steeper slope in the outer parts. Comment: 6 figures, 1 table, accepted for publication in MNRAS letters04/2010; -
Article: Thick gas discs in faint dwarf galaxies
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ABSTRACT: We determine the intrinsic axial ratio distribution of the 'gas' disks of extremely faint M_B > -14.5 dwarf irregular galaxies. We start with the measured (beam corrected) distribution of apparent axial ratios in the HI 21cm images of dwarf irregular galaxies observed as part of the Faint Irregular Galaxy GMRT Survey (FIGGS). Assuming that the disks can be approximated as oblate spheroids, the intrinsic axial ratio distribution can be obtained from the observed apparent axial ratio distribution. We use a couple of methods to do this, and our final results are based on using Lucy's deconvolution algorithm. This method is constrained to produce physically plausible distributions, and also has the added advantage of allowing for observational errors to be accounted for. While one might a priori expect that gas disks would be thin (because collisions between gas clouds would cause them to quickly settle down to a thin disk), we find that the HI disks of faint dwarf irregulars are quite thick, with mean axial ratio ~ 0.6. While this is substantially larger than the typical value of ~ 0.2 for the 'stellar' disks of large spiral galaxies, it is consistent with the much larger ratio of velocity dispersion to rotational velocity (sigma/v_c) in dwarf galaxy HI disks as compared to that in spiral galaxies. Our findings have implications for studies of the mass distribution in and the Tully - Fisher relation for faint dwarf irregular galaxies, where it is often assumed that the gas is in a thin disk. Comment: 5 pages, 4 figures. Minor changes in revised version. The definitive version is available at www.blackwell-synergy.com.02/2010; -
Article: HI 21 cm opacity fluctuations power spectra towards Cassiopeia A
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ABSTRACT: The angular power spectrum of HI 21 cm opacity fluctuations is a useful statistic for quantifying the observed opacity fluctuations as well as for comparing these with theoretical models. We present here the HI 21 cm opacity fluctuation power spectrum towards the supernova remnant Cas A from interferometric data with spacial resolution of 5" and spectral resolution of 0.4 km/s. The power spectrum has been estimated using a simple but robust visibility based technique. We find that the power spectrum is well fit by a power law P_tau(U) = U^{alpha} with a power law index of alpha ~ -2.86 +/- 0.10 (3 sigma error) over the scales of 0.07 - 2.3 pc for the gas in the Perseus spiral arm and 0.002 - 0.07 pc (480 - 15730 au) for that in the Local arm. This estimated power law index is consistent with earlier observational results based on both HI emission over larger scales and absorption studies over a similar range of scales. We do not detect any statistically significant change in the power law index with the velocity width of the frequency channels. This constrains the power law index of the velocity structure function to be beta = 0.2 +/- 0.6 (3 sigma error). Comment: 5 pages, 3 figures. Corrected minor typos. Accepted for publication in MNRAS Letters. The definitive version is available at http://www.blackwell-synergy.com/02/2010; -
Article: A study of interstellar medium of dwarf galaxies using H i power spectrum analysis
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ABSTRACT: We estimate the power spectrum of H i intensity fluctuations for a sample of eight galaxies (seven dwarf and one spiral). The power spectrum can be fitted to a power-law for six of these galaxies, indicating turbulence is operational. The estimated best-fitting value for the slope ranges from ∼−1.5 (AND IV, NGC 628, UGC 4459 and GR 8) to ∼−2.6 (DDO 210 and NGC 3741). We interpret this bi-modality as being due to having effectively 2D turbulence on length-scales much larger than the scale-height of the galaxy disc and 3D otherwise. This allows us to use the estimated slope to set bounds on the scale-heights of the face-on galaxies in our sample. We also find that the power-law slope remains constant as we increase the channel thickness for all these galaxies, suggesting that the fluctuations in H i intensity are due to density fluctuations and not velocity fluctuations, or that the slope of the velocity structure function is ∼0. Finally, for the four galaxies with ‘2D turbulence’ we find that the slope correlates with the star formation rate (SFR) per unit area, with larger SFRs leading to steeper power laws. Given our small sample size, this result needs to be confirmed with a larger sample.Monthly Notices of the Royal Astronomical Society 08/2009; 398(2):887 - 897. · 4.90 Impact Factor -
Article: The scaleheight of NGC 1058 measured from its H i power spectrum
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ABSTRACT: We have measured the H i power spectrum of the nearly face-on spiral galaxy NGC 1058 from radio-interferometric observations using a visibility based estimator. The power spectrum is well fitted by two different power laws P(U) =AUα, one with α=−2.5 ± 0.6 at small length-scales (600 pc to 1.5 kpc) and another with α=−1.0 ± 0.2 at large length-scales (1.5–10.0 kpc). We interpret this change in the slope of the power spectrum as a transition from 3D turbulence at small length-scales to 2D turbulence in the plane of the galaxy's disc at large length-scales. We use the observed break in the power spectrum to estimate the galaxy's scaleheight, which we find to be 490 ± 90 pc.Monthly Notices of the Royal Astronomical Society Letters 07/2009; 397(1):L60 - L63. -
Article: Probing Turbulence in the Interstellar Medium of Galaxies
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ABSTRACT: The power spectrum of HI intensity fluctuation of the interstellar medium carries information of the turbulence dynamics therein. We present a method to estimate the power spectrum of HI intensity fluctuation using radio interferometric observations. The method involves correlating the visibilities in the u-v plane at different baselines. This method is particularly use full for evaluating the power spectrum for the faint dwarf galaxies. We apply this method to 3 spiral galaxies and 5 dwarf galaxies. The measured power spectrum seem to follow a power law P_HI(U) = A U^\alpha, suggesting turbulence to be operational. Further, depending on the slope of the power spectrum, we expect the presence of 2D and 3D turbulence in those galaxies. Comment: 5 pages, 1 figure05/2009; -
Article: Star formation in extremely faint dwarf galaxies
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ABSTRACT: We study the relationship between the gas column density (derived from GMRT 21 cm data) and the star formation rate surface density (derived from publicly available GALEX data) for a sample of 23 extremely faint dwarf irregular galaxies drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS). Our sample galaxies have a median HI mass of 2.8e07 solar masses and a median blue magnitude -13.2. We find that gas column density averaged over the star forming region of the disk lies below most estimates of the "threshold density" for star formation, and that the average star formation rate surface density for most of the galaxies is also lower than would be expected from the "Kennicutt-Schmidt" law (Kennnicutt 1998}. We also use our data to look for small scale (400 pc and 200 pc) correlations. At 400 pc linear resolution, for 18 of our 23 galaxies, we find that star formation rate surface density can be parametrized as having a power law dependence on gas column density, which varies accross the sample and is in general steeper than "Kennicutt-Schmidt" law. The power law relation holds until one reaches the sensitivity limit of the GALEX data, i.e. we find no evidence for a "threshold density" below which star formation is completely cut off. For the 5 galaxies for which a power law does not provide a good parametrization, there are substantial offsets between the UV bright regions and the HI high column density maps. At 200 pc resolution, the offsets between the peaks in the HI and UV images are more pronounced, and a power law parametrization is possible for only 5 of 10 galaxies.05/2009; -
Article: A Study of ISM of Dwarf Galaxies Using HI Power Spectrum Analysis
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ABSTRACT: We estimate the power spectrum of HI intensity fluctuations for a sample of 8 galaxies (7 dwarf and one spiral). The power spectrum can be fitted to a power law P_HI(U) = A U^\alpha for 6 of these galaxies, indicating turbulence is operational. The estimated best fit value for the slope ranges from ~ -1.5 (AND IV, NGC 628, UGC 4459 and GR 8) to ~ -2.6 (DDO 210 and NGC 3741). We interpret this bi-modality as being due to having effectively 2D turbulence on length scales much larger than the scale height of the galaxy disk and 3D otherwise. This allows us to use the estimated slope to set bounds on the scale heights of the face-on galaxies in our sample. We also find that the power law slope remains constant as we increase the channel thickness for all these galaxies, suggesting that the fluctuations in HI intensity are due to density fluctuations and not velocity fluctuations, or that the slope of the velocity structure function is ~ 0. Finally, for the four galaxies with "2D turbulence" we find that the slope \alpha correlates with the star formation rate per unit area, with larger star formation rates leading to steeper power laws. Given our small sample size this result needs to be confirmed with a larger sample. Comment: 24 pages 10 figures and 2 tables. Accepted for publication in Mon. Not. of Royal Astron. Soc05/2009; -
Article: The Scale Height of NGC 1058 Measured from its HI Power Spectrum
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ABSTRACT: We have measured the HI power spectrum of the nearly face-on spiral galaxy NGC 1058 from radio-interferometric observations using a visibility based estimator. The power spectrum is well fitted by two different power laws $P(U)=AU^{\alpha}$, one with $\alpha =- 2.5\pm 0.6$ at small length-scales $(600 {\rm pc} {\rm to} 1.5 {\rm kpc})$ and another with $\alpha =- 1.0\pm 0.2$ at large length-scales $(1.5 {\rm kpc} {\rm to} 10.0 {\rm kpc})$. We interpret this change in the slope of the power spectrum as a transition from 3D turbulence at small length-scales to 2D turbulence in the plane of the galaxy's disk at large length-scales. We use the observed break in the power spectrum to estimate the galaxy's scale-height, which we find to be $ 490 \pm 90 $ pc. Comment: Accepted for publication in Mon. Not. of Royal Astron. Soc. Letters05/2009; -
Article: Magnetohydrodynamic turbulence in supernova remnants
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ABSTRACT: We present estimates of the angular power spectra of the synchrotron radiation intensity fluctuations at 6 and 20 cm for the shell type supernova remnant Cas A and the filled-centre Crab supernova remnant. We find that the intensity fluctuations of both sources have a power law power spectrum with index -3.24 +/- 0.03. This power law power spectrum is consistent with the magnetohydrodynamic turbulence in the synchrotron emitting plasma. For Cas A, there is a break in the power spectrum and the power law index changes from -3.2 to -2.2 at large angular scale. This transition occurs at an angular scale that corresponds to the shell thickness of Cas A. We interpret this as a transition from three dimensional turbulence to two dimensional turbulence on scales that are respectively smaller and larger than the shell thickness. Comment: 6 pages, 1 table, 5 figures. Corrected minor typos. Accepted for publication in MNRAS Letters. The definitive version will be available at http://www.blackwell-synergy.com/11/2008; -
Article: Foregrounds for redshifted 21‐cm studies of reionization: Giant Meter Wave Radio Telescope 153‐MHz observations
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ABSTRACT: Foreground subtraction is the biggest challenge for future redshifted 21-cm observations to probe reionization. We use a short Giant Meter Wave Radio Telescope (GMRT) observation at 153 MHz to characterize the statistical properties of the background radiation across ∼1° to subarcmin angular scales, and across a frequency band of 5 MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum Cl. We present the results obtained from using relatively unsophisticated, conventional data calibration procedures. We find that even fairly simple-minded calibration allows one to estimate the visibility correlation function at a given frequency V2(U, 0). From our observations, we find that V2(U, 0) is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies κ(U, Δν) seems to be much more sensitive to calibration errors. We find a rapid decline in κ(U, Δν), in contrast with the prediction of less than 1 per cent variation across 2.5 MHz. In this case, however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures.Monthly Notices of the Royal Astronomical Society 04/2008; 385(4):2166 - 2174. · 4.90 Impact Factor -
Article: Turbulence measurements from HI absorption spectra
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ABSTRACT: We use the millennium Arecibo 21 cm absorption-line survey measurements to examine the issue of the non-thermal contribution to the observed Galactic HI line widths. If we assume a simple, constant pressure model for the HI in the Galaxy, we find that the non-thermal contribution to the line width, v_{nt} scales as v^2_{nt} \propto l^{\alpha}, for v_{nt} larger than \sim 0.7 km s^{-1}. Here l is a derived length scale and \alpha \sim 0.7 \pm 0.1. This is consistent with what one would expect from a turbulent medium with a Kolmogorov scaling. Such a scaling is also predicted by theoretical models and numerical simulations of turbulence in a magnetized medium. For non-thermal line widths narrower than \sim 0.7 km s^{-1}, this scaling breaks down, and we find that the likely reason is ambiguities arising from Gaussian decomposition of intrinsically narrow, blended lines. We use the above estimate of the non-thermal contribution to the line width to determine corrected HI kinetic temperature. The new limits that we obtain imply that a significantly smaller (\sim 40% as opposed to 60%) fraction of the atomic interstellar medium in our Galaxy is in the warm neutral medium phase.04/2008;
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2009
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University of Cambridge
- Institute of Astronomy
Cambridge, ENG, United Kingdom
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2007
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University of Pune
Pune, State of Maharashtra, India
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