HI signal from re-ionization epoch

Abstract

We investigate the all-sky signal in redshifted atomic hydrogen (HI) line from the re-ionization epoch. We model the phase of re-ionization as multiple point sources that carve out spherical Stromgren spheres. We study ionization histories compatible with Wilkinson Microwave Anisotropy Probe (WMAP) observation. The Lyman α and soft X-ray emission from these sources are taken into account for studying the HI signal. HI can be observed in both emission and absorption depending on the ratio of Lyman α to ionizing flux and the spectrum of the radiation in soft X-ray. 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 follows. (i) The pre-ionized HI can be seen in absorption for ν~= 10-40 MHz; the maximum signal strength is ~=70-100 mK. (ii) A sharp absorption feature of width <~5 MHz might be observed in the frequency range ~=50-100 MHz, depending on the re-ionization history. The strength of the signal is proportional to the ratio of the Lyman α and the hydrogen-ionizing flux and the spectral index of the radiation field in soft X-ray. (iii) At larger frequencies, HI is seen in emission with peak frequency between 60 and 100MHz, depending on the ionization history of the Universe; the peak strength of this signal is ~=50 mK. From Fisher matrix analysis, we compute the precision with which the parameters of the model can be estimated from a future experiment as follows: (i) the pre-reionization signal can constrain a region in the Ωbh2-Ωmh2 plane (ii) HI observed in emission can be used to give precise, <~1 per cent, measurement of the evolution of the ionization fraction in the Universe and (iii) 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 a reasonably precise measurement of the fraction of the Universe heated by soft X-ray photons.