# The Escape Fraction of Ionizing Photons During the Epoch of Reionization: observability with the Square Kilometre Array

## TL;DR

This paper develops a self-consistent model linking the escape fraction of ionizing photons to galaxy evolution during reionization and assesses its observability with the Square Kilometre Array, highlighting potential for constraining this elusive parameter.

## Contribution

It introduces a physically motivated model of $f_{esc}$ coupled with galaxy feedback and explores its impact on 21cm power spectrum evolution for the first time.

## Key findings

- Changing $f_{esc}$ models significantly affects large-scale 21cm power.
- SKA can distinguish between models with 200 hours of observation at specific redshifts.
- Multiple 800-hour observations are needed to constrain $f_{esc}$ across a broader redshift range.

## Abstract

One of the most important parameters in characterizing the Epoch of Reionization, the escape fraction of ionizing photons, $f_\mathrm{esc}$, remains unconstrained both observationally and theoretically. With recent work highlighting the impact of galaxy-scale feedback on the instantaneous value of $f_\mathrm{esc}$, it is important to develop a model in which reionization is self-consistently coupled to galaxy evolution. In this work, we present such a model and explore how physically motivated functional forms of $f_\mathrm{esc}$ affect the evolution of ionized hydrogen within the intergalactic medium. Using the $21$cm power spectrum evolution, we investigate the likelihood of observationally distinguishing between a constant $f_\mathrm{esc}$ and other models that depend upon different forms of galaxy feedback. We find that changing the underlying connection between $f_\mathrm{esc}$ and galaxy feedback drastically alters the large-scale $21$cm power. The upcoming Square Kilometre Array Low Frequency instrument possesses the sensitivity to differentiate between our models at a fixed optical depth, requiring only $200$ hours of integration time focused on redshifts $z = 7.5-8.5$. Generalizing these results to account for a varying optical depth will require multiple $800$ hour observations spanning redshifts $z = 7-10$. This presents an exciting opportunity to observationally constrain one of the most elusive parameters during the Epoch of Reionization.