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Using the astraeus (seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N -body dark matter simUlationS) framework, we explore the impact of environmental density and radiative feedback on the assembly of galaxies and their host haloes during the Epoch of Reionization. The astraeus framework allows us to study the evolution of galaxies with masses (⁠|$10^{8.2}\,\rm M_\odot \lt M_{\rm h}\lt 10^{13}\,\rm M_\odot$|⁠) in a wide variety of environment [−0.5 < log (1 + δ) < 1.3 averaged over (2 cMpc)3]. We find that: (i) there exists a mass- and redshift- dependent 'characteristic' environment [ |${\rm log}\, (1+\delta _a(M_{\rm h}, z)) = 0.021\times (M_{\rm h}/\rm M_\odot)^{0.16} + 0.07 z -1.12$|⁠ , up to |$z$| ∼ 10] at which galaxies are most efficient at accreting dark matter, e.g at a rate of |$0.2{{\ \rm per\ cent}}$| of their mass every Myr at |$z$|  = 5; (ii) the number of minor and major mergers and their contributions to the dark matter assembly increases with halo mass at all redshifts and is mostly independent of the environment; (iii) at |$z$|  = 5 minor mergers contribute slightly more (by up to |$\sim 10{{\ \rm per\ cent}}$|⁠) to the dark matter assembly while for the stellar assembly, major mergers dominate the contribution from minor mergers for |$M_{\rm h}\lesssim 10^{11.5}\rm\, M_\odot$| galaxies; (iv) radiative feedback quenches star formation more in low-mass galaxies (⁠|$M_{\rm h}\lesssim 10^{9.5}\rm\, M_\odot$|⁠) in over-dense environments [log(1 + δ) > 0.5]; dominated by their major branch, this yields star formation histories biased towards older ages with a slower redshift evolution. [ABSTRACT FROM AUTHOR]

Low-mass galaxies are thought to provide the bulk of the ionizing radiation necessary to reionize the Universe. The amount of photons escaping the galaxies is poorly constrained theoretically, and difficult to measure observationally. Yet it is an essential parameter of reionization models. We study in detail how ionizing radiation can leak from high-redshift galaxies. For this purpose, we use a series of high-resolution radiation hydrodynamics simulations, zooming on three dwarf galaxies in a cosmological context. We find that the energy and momentum input from the supernova explosions has a pivotal role in regulating the escape fraction by disrupting dense star-forming clumps, and clearing sightlines in the halo. In the absence of supernovae, photons are absorbed very locally, within the birth clouds of massive stars. We follow the time evolution of the escape fraction and find that it can vary by more than six orders of magnitude. This explains the large scatter in the value of the escape fraction found by previous studies. This fast variability also impacts the observability of the sources of reionization: a survey even as deep as M1500 = -14 would miss about half of the underlying population of Lyman-continuum emitters. [ABSTRACT FROM AUTHOR]