The impact of H i in galaxies on 21-cm intensity fluctuations during the reionization epoch

Abstract

We investigate the impact of neutral hydrogen (H i) in galaxies on the statistics of 21-cm fluctuations using seminumerical modelling. Following the reionization of hydrogen, the H i content of the Universe is dominated by damped absorption systems (DLAs), with a cosmic density in H i that is observed to be constant at a level equal to ∼2 per cent of the cosmic baryon density from z ∼ 1 to z ∼ 5. We show that extrapolation of this constant fraction into the reionization epoch results in a reduction in the amplitude of 21-cm fluctuations over a range of spatial scales. We further find that consideration of H i in galaxies/DLAs reduces the prominence of the H ii region induced shoulder in the 21-cm power spectrum (PS), and hence modifies the scale dependence of 21-cm fluctuations. We also estimate the 21-cm-galaxy cross PS and show that the cross PS changes sign on scales corresponding to the H ii regions. From consideration of the sensitivity for forthcoming low-frequency arrays, we find that the effects of H i in galaxies/DLAs on the statistics of 21-cm fluctuations will be significant with respect to the precision of a PS or cross PS measurement. In addition, since overdense regions are reionized first we demonstrate that the cross-correlation between galaxies and 21-cm emission changes sign at the end of the reionization era, providing an alternative avenue to pinpoint the end of reionization. The sum of our analysis indicates that the H i content of the galaxies that reionize the universe will need to be considered in detailed modelling of the 21-cm intensity PS in order to correctly interpret measurements from forthcoming low-frequency arrays. Show more