Variation in the escape fraction of ionizing photons from galaxies and the redshifted 21-cm power spectrum during reionization

Abstract

The observed power spectrum of redshifted 21-cm fluctuations is 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 study the effect of mass- and redshift-dependent escape fractions of ionizing radiation on the ability of forthcoming experiments to constrain galaxy formation via the redshifted 21-cm power spectrum. We use a model for reionization which combines the hierarchical galaxy formation model GALFORM implemented within the Millennium-II dark matter simulation, with a seminumerical scheme to describe the resulting ionization structure. Using this model we show that the structure and distribution of ionized regions at fixed neutral fraction, and hence the slope and amplitude of the 21-cm power spectrum, are dependent on the variation of ionizing photon escape fraction with galaxy mass and redshift. However, we find that the influence of the unknown escape fraction and its evolution is smaller than the dominant astrophysical effect provided by supernovae feedback strength in high-redshift galaxies. The unknown escape fraction of ionizing radiation from galaxies is therefore unlikely to prevent measurement of the properties of high-redshift star formation using observations of the 21-cm power spectrum.