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We explore how the characteristics of the cross-correlation functions between the 21-cm emission from the spin-flip transition of neutral hydrogen (H  |$\scriptstyle \rm I $|⁠) and early Lyman α (Ly α) radiation emitting galaxies (Ly α emitters, LAEs) depend on the reionization history and topology and the simulated volume. For this purpose, we develop an analytic expression for the 21-cm-LAE cross-correlation function and compare it to results derived from different astraeus and 21cmfast reionization simulations covering a physically plausible range of scenarios where either low-mass (≲ 109.5 M⊙) or massive (≳ 109.5 M⊙) galaxies drive reionization. Our key findings are: (i) the negative small-scale (≲  2 cMpc) cross-correlation amplitude scales with the intergalactic medium's (IGM) average H  |$\scriptstyle \rm I $| fraction (〈χ H i 〉) and spin-temperature weighted overdensity in neutral regions (〈1 + δ〉 H i ); (ii) the inversion point of the cross-correlation function traces the peak of the size distribution of ionized regions around LAEs; (iii) the cross-correlation amplitude at small scales is sensitive to the reionization topology, with its anticorrelation or correlation decreasing the stronger the ionizing emissivity of the underlying galaxy population is correlated to the cosmic web gas distribution (i.e. the more low-mass galaxies drive reionization); (iv) the required simulation volume to not underpredict the 21-cm-LAE anticorrelation amplitude when the cross-correlation is derived via the cross-power spectrum rises as the size of ionized regions and their variance increases. Our analytic expression can serve two purposes: to test whether simulation volumes are sufficiently large, and to act as a fitting function when cross-correlating future 21-cm signal Square Kilometre Array and LAE galaxy observations. [ABSTRACT FROM AUTHOR]

We use the astraeus framework to investigate how the visibility and spatial distribution of Lyman-α (Lyα) emitters (LAEs) during reionization is sensitive to a halo mass-dependent fraction of ionizing radiation escaping from the galactic environment (f esc) and the ionization topology. To this end, we consider the two physically plausible bracketing scenarios of f esc increasing and decreasing with rising halo mass. We derive the corresponding observed Lyα luminosities of galaxies for three different analytic Lyα line profiles and associated Lyα escape fraction (⁠|$f_\mathrm{esc}^\mathrm{Ly\alpha }$|⁠) models: importantly, we introduce two novel analytic Lyα line profile models that describe the surrounding interstellar medium (ISM) as dusty gas clumps. They are based on parameterizing results from radiative transfer simulations, with one of them relating |$f_\mathrm{esc}^\mathrm{Ly\alpha }$| to f esc by assuming the ISM of being interspersed with low-density tunnels. Our key findings are: (i) for dusty gas clumps, the Lyα line profile develops from a central to double peak dominated profile as a galaxy's halo mass increases; (ii) LAEs are galaxies with |$M_h\gtrsim 10^{10}\, {\rm \rm M_\odot }$| located in overdense and highly ionized regions; (iii) for this reason, the spatial distribution of LAEs is primarily sensitive to the global ionization fraction and only weakly in second-order to the ionization topology or a halo mass-dependent f esc; (iv) furthermore, as the observed Lyα luminosity functions reflect the Lyα emission from more massive galaxies, there is a degeneracy between the f esc-dependent intrinsic Lyα luminosity and the Lyα attenuation by dust in the ISM if f esc does not exceed |$\sim 50~{{\ \rm per \, cent}}$|⁠. [ABSTRACT FROM AUTHOR]

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]

In this work, the sixth of a series, we use the seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N -body dark-matter simUlationS (astraeus) framework to investigate the nature of the sources that reionized the Universe. We extend astraeus , which already couples a galaxy formation semi-analytical model with a detailed seminumerical reionization scheme, to include a model for black-hole formation, growth, and the production of ionizing radiation from associated active galactic nuclei (AGNs). We calibrate our fiducial AGN model to reproduce the bolometric luminosity function at z ≃ 5, and explore the role of the resulting AGN population in reionizing the Universe. We find that in all the models yielding a reasonable AGN luminosity function, galaxies dominate overwhelmingly the ionizing budget during the Epoch of Reionization, with AGN accounting for 1–10 per cent of the ionizing budget at z  = 6 and starting to play a role only below z ≲ 5. [ABSTRACT FROM AUTHOR]

Recent deep surveys have unravelled a population of faint active galactic nuclei (AGNs) in the high-redshift Universe, leading to various discussions on their nature and their role during the Epoch of Reionization. We use cosmological radiation hydrodynamics simulations of a bright galaxy at z ∼ 6 (⁠|${M_\star } \gtrsim 10^{10}\, {\rm M}_{\odot }$|⁠) hosting an actively growing supermassive black hole to study the properties of these objects. In particular, we study how the black hole and the galaxy coevolve and what is the relative contribution of the AGNs and of the stellar populations to the luminosity budget of the system. We find that the feedback from the AGN has no strong effect on the properties of the galaxy, and does not increase the total ionizing luminosity of the host. The average escape fraction of our galaxy is around |$f_{\rm esc} \sim 5{{\ \rm per\ cent}}$|⁠. While our galaxy would be selected as an AGN in deep X-ray surveys, most of the ultraviolet (UV) luminosity is originating from stellar populations. This confirms that there is a transition in the galaxy population from star-forming galaxies to quasar hosts, with bright Lyman-break galaxies with M UV around −22 falling in the overlap region. Our results also suggest that faint AGNs do not contribute significantly to reionizing the Universe. [ABSTRACT FROM AUTHOR]