The structure of reionization in hierarchical galaxy formation models

Abstract

Understanding the epoch of reionization and the properties of the first galaxies represents an important goal for modern cosmology. The structure of reionization and hence the observed power spectrum of redshifted 21-cm fluctuations are known to be sensitive to the astrophysical properties of the galaxies that drove reionization. Thus, detailed measurements of the 21-cm power spectrum and its evolution could lead to measurements of the properties of early galaxies that are otherwise inaccessible. In this paper, we make predictions for the ionized structure during reionization and the 21-cm power spectrum based on detailed models of galaxy formation. We combine the semi-analytic galform model implemented within the Millennium-II dark matter simulation, with a semi-numerical scheme to describe the resulting ionization structure. Semi-analytic models based on the Millennium-II Simulation follow the properties of galaxies within haloes of mass greater than ∼1.4 × 108 M⊙ at z > 6, corresponding to the faint sources thought to dominate reionization. Using these models we show that the details of supernovae (SNe) and radiative feedback affect the structure and distribution of ionized regions, and hence the slope and amplitude of the 21-cm power spectrum. These results indicate that forthcoming measurements of the 21-cm power spectrum could be used to uncover details of early galaxy formation. We find that the strength of SN feedback is the dominant effect governing the evolution of structure during reionization. In particular, we show SN feedback to be more important than radiative feedback, the presence of which we find does not influence either the total stellar mass or overall ionizing photon budget. Thus, if SN feedback is effective at suppressing star formation in high-redshift galaxies, we find that photoionization feedback does not lead to self-regulation of the reionization process as has been thought.