Astraeus III: The environment and physical properties of reionization sources

Abstract

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 $M_h\simeq 10^{9-11}$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 ($M_h\lesssim 10^{9.5}$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 ($M_h\lesssim 10^9$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).

14 pages, 13 figures, accepted for publication in MNRAS