Astraeus III: The environment and physical properties of reionization
  sources
  In this work, we use the {\sc astraeus} (seminumerical rAdiative tranSfer
coupling of galaxy formaTion and Reionization in N-body dArk mattEr
simUlationS) framework which couples galaxy formation and reionization in the
first billion years. Exploring a number of models for reionization feedback and
the escape fraction of ionizing radiation from the galactic environment
($f_\mathrm{esc}$), we quantify how the contribution of star-forming galaxies
{(with halo masses $M_h>10^{8.2}$M$_\odot$)} to reionization depends on the
radiative feedback model, $f_\mathrm{esc}$, and the environmental over-density.
Our key findings are: (i) for constant $f_\mathrm{esc}$ models,
intermediate-mass galaxies (with halo masses of $M_h\simeq10^{9-11}$M$_\odot$
and absolute UV magnitudes of $M_{UV} \sim -15$ to $-20$) in
intermediate-density regions drive reionization; (ii) scenarios where
$f_\mathrm{esc}$ increases with decreasing halo mass shift the galaxy
population driving reionization to lower-mass galaxies
($M_h\lesssim10^{9.5}$M$_\odot$) with lower luminosities ($M_{UV} \gtrsim-16$)
and over-densities; (iii) reionization imprints its topology on the ionizing
emissivity of low-mass galaxies ($M_h\lesssim10^{9}$M$_\odot$) through
radiative feedback. Low-mass galaxies experience a stronger suppression of star
formation by radiative feedback and show lower ionizing emissivities in
over-dense regions; (iv) a change in $f_\mathrm{esc}$ with galaxy properties
has the largest impact on the sources of reionization and their detectability,
with the radiative feedback strength and environmental over-density playing a
sub-dominant role; (v) JWST-surveys (with a limiting magnitude of $M_{UV} =
-16$) will be able to detect the galaxies providing $\sim 60-70\%$ ($\sim
10\%$) of reionization photons at $z=7$ for constant $f_\mathrm{esc}$ models
(scenarios where $f_\mathrm{esc}$ increases with decreasing halo mass).
