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

TL;DR

This study investigates how variations in the escape fraction of ionising photons from galaxies affect the 21cm power spectrum during reionization, finding that these variations have a smaller impact than supernova feedback effects.

Contribution

It introduces a combined galaxy formation and semi-numerical reionization model to analyze the impact of escape fraction variations on 21cm observations, highlighting their relative significance.

Findings

Escape fraction variation influences ionized region structure.

Supernova feedback has a larger effect than escape fraction evolution.

Measurement of high redshift galaxy properties remains feasible.

Abstract

The observed power spectrum of redshifted 21cm fluctuations is known to be sensitive to the astrophysical properties of the galaxies that drove reionization. Thus, detailed measurements of the 21cm 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 21cm 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 semi-numerical scheme to describe the resulting ionization structure. Using this model we show that the structure and distribution of ionised regions at fixed neutral fraction, and hence the slope and amplitude of the 21 cm power spectrum, is dependent on the variation of ionising 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 SNe 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 21cm power spectrum.