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Authors
Shiv K. Sethi
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We investigate the all-sky signal in redshifted atomic hydrogen (HI) line from the reionization epoch. HI can be observed in both emission and absorption depending on the ratio of Lyman-$alpha$ to ionizing flux and the spectrum of the radiation in soft xray. We also compute the signal from pre-reionization epoch and show that within the uncertainty in cosmological parameters, it is fairly robust. The main features of HI signal can be summarized as: (a) The pre-ionized HI can be seen in absorption for $ u simeq 10hbox{--}40 rm MHz$; the maximum signal strength is $simeq 70hbox{--}100 rm mK$. (b) A sharp absorption feature of width $la 5 rm MHz$ might be observed in the frequency range $simeq 50 hbox{--}100 rm MHz$, depending on the reionization history. The strength of the signal is proportional to the ratio of the Lyman-$alpha$ and the hydrogen-ionizing flux and the spectral index of the radiation field in soft xray (c) At larger frequencies, HI is seen in emission with peak frequency between $60hbox{--}100 rm MHz$, depending on the ionization history of the universe; the peak strength of this signal is $simeq 50 rm mK$. From Fisher matrix analysis, we compute the precision with which the parameters of the model can be estimated from a future experiment: (a) the pre-reionization signal can constrain a region in the $Omega_b h^2$--$Omega_m h^2$ plane (b) HI observed in emission can be used to give precise, $la 1 %$, measurement of the evolution of the ionization fraction in the universe, and (c) the transition region from absorption to emission can be used as a probe of the spectrum of ionizing sources; in particular, the HI signal in this regime can give reasonably precise measurement of the fraction of the universe heated by soft x-ray photons.
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The implication of primordial magnetic-field-induced structure formation for the HI signal from the epoch of reionization is studied. Using semi-analytic models, we compute both the density and ionization inhomogeneities in this scenario. We show that: (a) The global HI signal can only be seen in emission, unlike in the standard $Lambda$CDM models, (b) the density perturbations induced by primordial fields, leave distinctive signatures of the magnetic field Jeans length on the HI two-point correlation function, (c) the length scale of ionization inhomogeneities is $la 1 rm Mpc$. We find that the peak expected signal (two-point correlation function) is $simeq 10^{-4} rm K^2$ in the range of scales $0.5hbox{-}3 rm Mpc$ for magnetic field strength in the range $5 times 10^{-10} hbox{-}3 times 10^{-9} rm G$. We also discuss the detectability of the HI signal. The angular resolution of the on-going and planned radio interferometers allows one to probe only the largest magnetic field strengths that we consider. They have the sensitivity to detect the magnetic field-induced features. We show that thefuture SKA has both the angular resolution and the sensitivity to detect the magnetic field-induced signal in the entire range of magnetic field values we consider, in an integration time of one week.
The neutral hydrogen (HI) and its 21 cm line are promising probes to the reionization process of the intergalactic medium (IGM). To use this probe effectively, it is imperative to have a good understanding on how the neutral hydrogen traces the underlying matter distribution. Here we study this problem using semi-numerical modeling by combining the HI in the IGM and the HI from halos during the epoch of reionization (EoR), and investigate the evolution and the scale-dependence of the neutral fraction bias as well as the 21 cm line bias. We find that the neutral fraction bias on large scales is negative during reionization, and its absolute value on large scales increases during the early stage of reionization and then decreases during the late stage. During the late stage of reionization, there is a transition scale at which the HI bias transits from negative on large scales to positive on small scales, and this scale increases as the reionization proceeds to the end.
The detection of the Epoch of Reionization (EoR) in the redshifted 21-cm line is a challenging task. Here we formulate the detection of the EoR signal using the drift scan strategy. This method potentially has better instrumental stability as compared to the case where a single patch of sky is tracked. We demonstrate that the correlation time between measured visibilities could extend up to 1-2 hr for an interferometer array such as the Murchison Widefield Array (MWA), which has a wide primary beam. We estimate the EoR power based on cross-correlation of visibilities across time and show that the drift scan strategy is capable of the detection of the EoR signal with comparable/better signal-to-noise as compared to the tracking case. We also estimate the visibility correlation for a set of bright point sources and argue that the statistical inhomogeneity of bright point sources might allow their separation from the EoR signal.
Using a suite of detailed numerical simulations we estimate the level of anisotropy generated by the time evolution along the light cone of the 21cm signal from the epoch of reionization. Our simulations include the physics necessary to model the signal during both the late emission regime and the early absorption regime, namely X-ray and Lyman-band 3D radiative transfer in addition to the usual dynamics and ionizing UV transfer. The signal is analysed using correlation functions perpendicular and parallel to the line of sight (LOS). We reproduce general findings from previous theoretical studies: the overall amplitude of the correlations and the fact that the light cone anisotropy is visible only on large scales (100 cMpc). However, the detailed behaviour is different. At 3 different epochs, the amplitude of the correlations along and perpendicular to the LOS differ from each other, indicating anisotropy. These 3 epochs are associated with 3 events of the global reionization history: the overlap of ionized bubbles, the onset of mild heating by X-rays in regions around the sources, and the onset of efficient Lyman-alpha coupling in regions around the sources. A 20x20 deg^2 survey area may be necessary to mitigate sample variance when we use the directional correlation functions. On a 100 cMpc scale the light cone anisotropy dominates over the anisotropy generated by peculiar velocity gradients computed in the linear regime. By modelling instrumental noise and limited resolution, we find that the anisotropy should be easily detectable by the SKA, assuming perfect foreground removal, the limiting factor being a large enough survey size. In the case of the LOFAR, it is likely that only first anisotropy episode will fall in the observing frequency range and will be detectable only if sample variance is much reduced (i.e. a larger than 20x20 deg^2 survey, which is not presently planned).
The motion of the solar system with respect to the cosmic rest frame modulates the monopole of the Epoch of Reionization 21-cm signal into a dipole. This dipole has a characteristic frequency dependence that is dominated by the frequency derivative of the monopole signal. We argue that although the signal is weaker by a factor of $sim100$, there are significant benefits in measuring the dipole. Most importantly, the direction of the cosmic velocity vector is known exquisitely well from the cosmic microwave background and is not aligned with the galaxy velocity vector that modulates the foreground monopole. Moreover, an experiment designed to measure a dipole can rely on differencing patches of the sky rather than making an absolute signal measurement, which helps with some systematic effects.
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