Search

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]

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]

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]

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 present NOEMA CO (2–1) line and ALMA 870 µm continuum observations of a main-sequence galaxy at z = 1.38. The galaxy was initially deemed a "gas giant" based on the gas mass derived from sub-mm continuum (log(Mgas/M⊙) = 11.20 ± 0.20), however, the gas mass derived from CO (2−1) luminosity brings the gas mass value down to a level that is consistent with typical values for star-forming galaxies at that redshift (log(Mgas/M⊙) = 10.84 ± 0.03). Meanwhile, the dust-to-stellar mass ratio remains elevated above the scaling relations by a factor of 5. In this work, we explore the potential physical picture and consider an underestimated stellar mass and optically thick dust as possible explanations. Based on the updated gas-to-stellar mass ratio, we can rule out the former; while the latter may indeed contribute to the overestimation of the dust mass, it is not sufficient enough to explain the observed physical picture overall. Instead, other plausible explanations include enhanced HI reservoirs, an unusually high metallicity, or the presence of an optically dark, dusty contaminant. Using the ALMA data at 870 µm coupled with HST/ACS imaging, we find an extended morphology in dust continuum and clumpy star-formation in rest-frame UV in this galaxy. In addition, a tentative ~10 kpc dusty arm is found to be bridging the galaxy center and a clump in F814W image. The galaxy shows levels of dust obscuration similar to the so-called HST-dark galaxies at higher redshifts, thus falling into the optically faint and dark JWST color-color selection at z > 2. It is therefore possible that our object may stand as a low-z analog of the HST-dark populations. This galaxy serves as a caveat to the gas masses based on the continuum alone, with a larger sample required to unveil the full picture. [ABSTRACT FROM AUTHOR]

The Ly  α line is a powerful probe of distant galaxies, which contains information about inflowing/outflowing gas through which Ly  α photons scatter. To develop our understanding of this probe, we post-process a zoom-in radiation-hydrodynamics simulation of a low-mass (M star ∼ 109 M⊙) galaxy to construct 22 500 mock spectra in 300 directions from z  = 3 to 4. Remarkably, we show that one galaxy can reproduce the variety of a large sample of spectroscopically observed Ly  α line profiles. While most mock spectra exhibit double-peak profiles with a dominant red peak, their shapes cover a large parameter space in terms of peak velocities, peak separation, and flux ratio. This diversity originates from radiative transfer effects at interstellar medium and circum-galactic medium (CGM) scales, and depends on galaxy inclination and evolutionary phase. Red-dominated lines preferentially arise in face-on directions during post-starburst outflows and are bright. Conversely, accretion phases usually yield symmetric double peaks in the edge-on direction and are fainter. While resonant scattering effects at <0.2 × R vir are responsible for the broadening and velocity shift of the red peak, the extended CGM acts as a screen and impacts the observed peak separation. The ability of simulations to reproduce observed Ly  α profiles and link their properties with galaxy physical parameters offers new perspectives to use Ly  α to constrain the mechanisms that regulate galaxy formation and evolution. Notably, our study implies that deeper Ly  α surveys may unveil a new population of blue-dominated lines tracing inflowing gas. [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]

Massive black hole (MBH) coalescences are powerful sources of low-frequency gravitational waves. To study these events in the cosmological context, we need to trace the large-scale structure and cosmic evolution of a statistical population of galaxies, from dim dwarfs to bright galaxies. To cover such a large range of galaxy masses, we analyse two complementary simulations: horizon-AGN with a large volume and low resolution that tracks the high-mass (⁠|$\gt 10^7\, {\rm M_\odot }$|⁠) MBH population, and NewHorizon with a smaller volume but higher resolution that traces the low-mass (|$\lt 10^7\, {\rm M_\odot }$|⁠) MBH population. While Horizon-AGN can be used to estimate the rate of inspirals for pulsar timing arrays, NewHorizon  can investigate MBH mergers in a statistical sample of dwarf galaxies for LISA, which is sensitive to low-mass MBHs. We use the same method to analyse the two simulations, post-processing MBH dynamics to account for time delays mostly determined by dynamical friction and stellar hardening. In both simulations, MBHs typically merge long after galaxies do, so that the galaxy morphology at the time of the MBH merger is no longer determined by the structural disturbances engendered by the galaxy merger from which the MBH coalescence has originated. These time delays cause a loss of high- z MBH coalescences, shifting the peak of the MBH merger rate to z ∼ 1–2. This study shows how tracking MBH mergers in low-mass galaxies is crucial to probing the MBH merger rate for LISA and investigate the properties of the host galaxies. [ABSTRACT FROM AUTHOR]

Super-Eddington accretion on to massive black hole seeds may be commonplace in the early Universe, where the conditions exist for rapid accretion. Direct-collapse black holes are often invoked as a possible solution to the observation of supermassive black holes (SMBHs) in the pre-reionization Universe. We investigate here how feedback, mainly in the form of bipolar jets, from super-Eddington accreting seed black holes will affect their subsequent growth. We find that, nearly independently of the mass loading of the bipolar jets, the violent outflows generated by the jets evacuate a region of approximately 0.1 pc surrounding the black hole seed. However, the jet outflows are unable to break free of the halo and their impact is limited to the immediate vicinity of the black hole. The outflows suppress any accretion for approximately a dynamical time. The gas then cools, recombines, and falls back to the centre, where high accretion rates are again observed. The overall effect is to create an effective accretion rate with values of between 0.1 and 0.5 times the Eddington rate. If this episodic accretion rate is maintained for order 500 million years, then the black hole will increase in mass by a factor of between 3 and 300 but far short of the factor of 104 required for the seeds to become the SMBHs observed at z > 6. Therefore, direct-collapse black holes born into atomic cooling haloes and which experience strong negative mechanical feedback will require external influences (e.g. rapid major mergers with other haloes) to promote efficient accretion and reach SMBH masses within a few hundred million years. [ABSTRACT FROM AUTHOR]