Astraeus - III. The environment and physical properties of reionization sources

In this work, we use the ASTRAEUS (seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dArk mattEr simUlationS) framework that 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_esc), we quantify how the contribution of star-forming galaxies (with halo masses $M_\mathrm{ h}\gt 10^{8.2}\, {\rm \rm M_\odot }$) to reionization depends on the radiative feedback model, f_esc, and the environmental overdensity. Our key findings are: (i) for constant f_esc models, intermediate-mass galaxies (with halo masses of $M_\mathrm{ h}\simeq 10^{9-11}\, {\rm \rm M_\odot }$ and absolute UV magnitudes of M_UV ~ -15 to -20) in intermediate-density regions (with overdensity log₁₀(1 + δ) ~ 0-0.8 on a 2 comoving Mpc spatial scale) drive reionization; (ii) scenarios where f_esc increases with decreasing halo mass shift, the galaxy population driving reionization to lower mass galaxies ($M_\mathrm{ h}\lesssim 10^{9.5}\, {\rm \rm M_\odot }$) with lower luminosities (M_UV ≳ -16) and overdensities [log₁₀(1 + δ) ~ 0-0.5 on a 2 comoving Mpc spatial scale]; (iii) reionization imprints its topology on the ionizing emissivity of low-mass galaxies ($M_h\lesssim 10^{9}\, {\rm \rm M_\odot }$] through radiative feedback. Low-mass galaxies experience a stronger suppression of star formation by radiative feedback and show lower ionizing emissivities in overdense regions; (iv) a change in f_esc with galaxy properties has the largest impact on the sources of reionization and their detectability, with the radiative feedback strength and environmental overdensity playing a sub-dominant role; (v) James Webb Space Telescope-surveys (with a limiting magnitude of M_UV = -16) will be able to detect the galaxies providing ${\sim}60{-}70{{\ \rm per\ cent}}$ (${\sim}10{{\ \rm per\ cent}}$) of reionization photons at z = 7 for constant f_esc models (scenarios where f_esc increases with decreasing halo mass).