The cosmic microwave background radiation fluctuations from H i perturbations prior to reionization

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 06/2004; 352(1):142 - 146. DOI: 10.1111/j.1365-2966.2004.07907.x
Source: arXiv


Loeb and Zaldarriaga have recently proposed that observations of the cosmic microwave background radiation (CMBR) brightness temperature fluctuations produced by H i inhomogeneities prior to reionization hold the promise of probing the primordial power spectrum to a hitherto unprecedented level of accuracy. This requires a precise quantification of the relation between density perturbations and brightness temperature fluctuations. Brightness temperature fluctuations arise from two sources: (1) fluctuations in the spin temperature, and (2) fluctuations in the H i optical depth, both of which are caused by density perturbations. For the spin temperature, we investigate in detail its evolution in the presence of H i fluctuations. For the optical depth, we find that it is affected by density perturbations both directly and through peculiar velocities which move the absorption features around in frequency. The latter effect, which has not been included in earlier studies, is similar to the redshift space distortion seen in galaxy surveys and this can cause changes of 50 per cent or more in the brightness temperature fluctuations.

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    ABSTRACT: Recent theoretical developments for observing the Epoch of Reionization (EOR) have concentrated on the power spectrum signature of redshifted 21 cm emission. These studies have demonstrated the great potential of statistical EOR observations, however, the sensitivity calculations for proposed low frequency radio arrays have been highly approximate. The formalism developed for interferometric measurements of the cosmic microwave background can be extended to three dimensions to naturally incorporate the line-of-sight information inherent in the EOR signal. In this paper we demonstrate how to accurately calculate the EOR power spectrum sensitivity of an array, and develop scaling relationships which can be used to guide the design of EOR observatories. The implications for antenna distribution, antenna size, and correlator requirements on the EOR sensitivity are detailed. Comment: 7 pages, 3 figures, submitted to ApJ
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    ABSTRACT: Redshifted 21 cm radiation originating from the cosmological distribution of neutral hydrogen (HI) appears as a background radiation in low frequency radio observations. The angular and frequency domain fluctuations in this radiation carry information about cosmological structure formation. We propose that correlations between visibilities measured at different baselines and frequencies in radio-interferometric observations be used to quantify the statistical properties of these fluctuations. This has an inherent advantage over other statistical estimators in that it deals directly with the visibilities which are the primary quantities measured in radio-interferometric observations. Also, the visibility correlation has a very simple relation with power spectrum. We present estimates of the expected signal for nearly the entire post-recombination era, from the dark ages to the present epoch. The epoch of reionization, where the HI has a patchy distribution, has a distinct signature where the signal is determined by the size of the discrete ionized regions. The signal at other epochs, where the HI follows the dark matter, is determined largely by the power spectrum of dark matter fluctuations. The signal is strongest for baselines where the antenna separations are within a few hundred times the wavelength of observation, and an optimal strategy would preferentially sample these baselines. In the frequency domain, for most baselines the visibilities at two different frequencies are uncorrelated beyond \Delta \nu ~ 1 MHz, a signature which in principle would allow the HI signal to be easily distinguished from the continuum sources of contamination.
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    ABSTRACT: We re-examine the role of collisions in decoupling the HI 21-cm spin temperature from the cosmic microwave background (CMB). The cross section for de-exciting the 21-cm trasition in collisions with free electrons is more than 10 times larger than it is in collisions with other atoms. If the fraction of free electrons in the diffuse cosmic gas is between 10 and 30 per cent then collisions alone can decouple the spin temperature from the cosmic microwave background (CMB), even in moderately under-dense regions at z >15. This decoupling is especially important during the very early stages of re-ionization when a Ly-alpha continuum background had yet to be established. As a detailed example, we develop a semi-analytic model to quantify 21-cm emission signatures from a diffuse gas which is partially ionized at z>10 by an X-ray background. We find 21-cm differential brightness temperature fluctuations with a mean of ~8 mK and a rms value as large as 5 mK, for a frequency resolution bandwidth of 100 KHz and a beamsize of 3 arcmin. Another example where free electron-atom collisions are important is during the recombination of bubbles ionized by short-lived UV sources. When the ionized fraction in these bubbles drops to 10-20 per cent their differential temperature can be as high as 10 mK. We also consider the correlation of the 21-cm temperature as a function of frequency. By isotropy, a comparison with the angular projected correlation in the plane of the sky should yield an estimate for the matter density parameter, Omega_m, and the dark energy component. This variant of the Alcock-Paczyski test is applicable for any reionization scenario. Comment: version to appear in the MNRAS
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