50 research outputs found
High-redshift galaxies and black holes in the eyes of JWST: a population synthesis model from infrared to X-rays
The first billion years of the Universe is a pivotal time: stars, black holes
(BHs) and galaxies form and assemble, sowing the seeds of galaxies as we know
them today. Detecting, identifying and understand- ing the first galaxies and
BHs is one of the current observational and theoretical challenges in galaxy
formation. In this paper we present a population synthesis model aimed at
galaxies, BHs and Active Galactic Nuclei (AGNs) at high redshift. The model
builds a population based on empirical relations. Galaxies are characterized by
a spectral energy distribution determined by age and metallicity, and AGNs by a
spectral energy distribution determined by BH mass and accretion rate. We
validate the model against observational constraints, and then predict
properties of galaxies and AGN in other wavelength and/or luminosity ranges,
estimating the contamination of stellar populations (normal stars and high-mass
X-ray binaries) for AGN searches from the infrared to X-rays, and vice-versa
for galaxy searches. For high-redshift galaxies, with stellar ages < 1 Gyr, we
find that disentangling stellar and AGN emission is challenging at restframe
UV/optical wavelengths, while high-mass X-ray binaries become more important
sources of confusion in X-rays. We propose a color-color selection in JWST
bands to separate AGN vs star-dominated galaxies in photometric observations.
We also esti- mate the AGN contribution, with respect to massive, hot,
metal-poor stars, at driving high ionization lines, such as C IV and He II.
Finally, we test the influence of the minimum BH mass and occupa- tion fraction
of BHs in low mass galaxies on the restframe UV/near-IR and X-ray AGN
luminosity function.Comment: Accepted for publication in the Astrophysical Journa
Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity function
Recent deep surveys have unravelled a population of faint active galactic
nuclei (AGN) 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
() hosting an actively growing super-massive
black hole to study the properties of these objects. In particular, we study
how the black hole and the galaxy co-evolve and what is the relative
contribution of the AGN 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
. While our galaxy would be selected as an AGN in deep
X-ray surveys, most of the 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
(LBGs) with around -22 falling in the overlap region. Our results
also suggest that faint AGN do not contribute significantly to reionizing the
Universe.Comment: 10+1 pages, 10+1 figures. Accepted for publications in MNRAS. Key
figures: 5 and 1
Astraeus VII: The environmental-dependent assembly of galaxies in the Epoch of Reionization
Using the ASTRAEUS (semi-numerical 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 halos during the Epoch of Reionization. The
ASTRAEUS framework allows us to study the evolution of galaxies with masses
() in wide variety of
environment ( averaged over ). We find that : (i) there exists a mass- and redshift- dependent
"characteristic" environment (, up to ) at which
galaxies are most efficient at accreting dark matter, e.g at a rate of
of their mass every Myr at ; (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
minor mergers contribute slightly more (by up to ) to the dark
matter assembly while for the stellar assembly, major mergers dominate the
contribution from minor mergers for
galaxies; (iv) radiative feedback quenches star formation more in low-mass
galaxies () in over-dense environments
(); dominated by their major branch, this yields
star formation histories biased towards older ages with a slower redshift
evolution.Comment: 17 pages, 15 figures, submitted to MNRAS, comments welcome
Astraeus VIII: A new framework for Lyman- emitters applied to different reionisation scenarios
We use the {\sc astraeus} framework to investigate how the visibility and
spatial distribution of Lyman- (Ly) emitters (LAEs) during
reionisation is sensitive to a halo mass-dependent fraction of ionising
radiation escaping from the galactic environment () and the
ionisation topology. To this end, we consider the two physically plausible
bracketing scenarios of increasing and decreasing with rising
halo mass. We derive the corresponding observed Ly luminosities of
galaxies for three different analytic Ly line profiles and associated
Ly escape fraction () models:
importantly, we introduce two novel analytic Ly line profile models
that describe the surrounding interstellar medium (ISM) as dusty gas clumps.
They are based on parameterising results from radiative transfer simulations,
with one of them relating to
by assuming the ISM of being interspersed with low-density
tunnels. Our key findings are: (i) for dusty gas clumps, the Ly line
profile develops from a central to double peak profile as a galaxy's halo mass
increases; (ii) LAEs are galaxies with located in
overdense and highly ionised regions; (iii) for this reason, the spatial
distribution of LAEs is primarily sensitive to the global ionisation fraction
and only weakly in second-order to the ionisation topology or a halo
mass-dependent ; (iv) furthermore, as the observed Ly
luminosity functions reflect the Ly emission from more massive
galaxies, there is a degeneracy between the -dependent
intrinsic Ly luminosity and the Ly attenuation by dust in the
ISM if does not exceed .Comment: 25 pages, 9 figures; accepted for publication in MNRA
Astraeus - VI. Hierarchical assembly of AGN and their large-scale effect during the Epoch of Reionization
In this work, the sixth of a series, we use the ASTRAEUS (semi-numerical
rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dark
matter simUlationS) 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 semi-numerical reionization
scheme, to include a model for black hole formation, growth, and the production
of ionizing radiation from associated AGN (active galactic nuclei). 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% of the ionizing
budget at z=6 and starting to play a role only below z < 5.Comment: 15+2 pages, 14+1 figures, 1 table. Accepted in MNRA
On the general nature of 21cm-Lyman- emitters cross-correlations during reionisation
We explore how the characteristics of the cross-correlation functions between
the 21cm emission from the spin-flip transition of neutral hydrogen (HI) and
early Lyman- (Ly) radiation emitting galaxies (Ly
emitters, LAEs) depend on the reionisation history and topology and the
simulated volume. For this purpose, we develop an analytic expression for the
21cm-LAE cross-correlation function and compare it to results derived from
different Astraeus and 21cmFAST reionisation simulations covering a physically
plausible range of scenarios where either low-mass () or
massive () galaxies drive reionisation. Our key findings are:
(i) the negative small-scale ( cMpc) cross-correlation amplitude scales
with the intergalactic medium's (IGM) average HI fraction
() and spin-temperature weighted overdensity in
neutral regions (); (ii) the inversion
point of the cross-correlation function traces the peak of the size
distribution of ionised regions around LAEs; (iii) the cross-correlation
amplitude at small scales is sensitive to the reionisation topology, with its
anti-correlation or correlation decreasing the stronger the ionising emissivity
of the underlying galaxy population is correlated to the cosmic web gas
distribution (i.e. the more low-mass galaxies drive reionisation); (iv) the
required simulation volume to not underpredict the 21cm-LAE anti-correlation
amplitude when the cross-correlation is derived via the cross-power spectrum
rises as the size of ionised 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 21cm signal Square Kilometre Array and LAE galaxy observations.Comment: 13 pages, 5 figures; accepted for publication in MNRA
Simulating the diversity of shapes of the Lyman- line
The Ly line is a powerful probe of distant galaxies, which contains
information about inflowing/outflowing gas through which Ly photons
scatter. To develop our understanding of this probe, we post-process a zoom-in
radiation-hydrodynamics simulation of a low-mass ()
galaxy to construct 22500 mock spectra in 300 directions from to 4.
Remarkably, we show that one galaxy can reproduce the variety of a large sample
of spectroscopically observed Ly 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 ISM and 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 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 profiles and link their properties with galaxy physical parameters
offers new perspectives to use Ly to constrain the mechanisms that
regulate galaxy formation and evolution. Notably, our study implies that deeper
Ly surveys may unveil a new population of blue-dominated lines tracing
inflowing gas.Comment: Accepted for publication in MNRA
Reionization with galaxies and active galactic nuclei
In this work we investigate the properties of the sources that reionized the
intergalactic medium (IGM) in the high-redshift Universe. Using a
semi-analytical model aimed at reproducing galaxies and black holes in the
first 1.5 Gyr of the Universe, we revisit the relative role of star formation
and black hole accretion in producing ionizing photons that can escape into the
IGM. Both star formation and black hole accretion are regulated by supernova
feedback, resulting in black hole accretion being stunted in low-mass halos. We
explore a wide range of combinations for the escape fraction of ionizing
photons (redshift-dependent, constant and scaling with stellar mass) from both
star formation () and AGN () to find: (i) the ionizing budget is dominated by stellar
radiation from low stellar mass (
with the AGN contribution (driven by black holes in
galaxies) dominating at lower redshifts; (ii) AGN
only contribute to the cumulative ionizing emissivity by for
the models that match the observed reionization constraints; (iii) if the
stellar mass dependence of is shallower
than , at a transition stellar mass exists above
which AGN dominate the escaping ionizing photon production rate; (iv) the
transition stellar mass decreases with decreasing redshift. While AGN dominate
the escaping emissivity above the knee of the stellar mass function at , they take-over at stellar masses that are a tenth of the knee mass by
.Comment: Accepted to MNRA
Ultra-light Axions: Degeneracies with Massive Neutrinos and Forecasts for Future Cosmological Observations
A generic prediction of string theory is the existence of many axion fields.
It has recently been argued that many of these fields should be light and, like
the well known QCD axion, lead to observable cosmological consequences. In this
paper we study in detail the effect of the so-called string axiverse on large
scale structure, focusing on the morphology and evolution of density
perturbations, anisotropies in the cosmic microwave background and weak
gravitational lensing of distant galaxies. We quantify specific effects that
will arise from the presence of the axionic fields and highlight possible
degeneracies that may arise in the presence of massive neutrinos. We take
particular care understanding the different physical effects and scales that
come into play. We then forecast how the string axiverse may be constrained and
show that with a combination of different observations, it should be possible
to detect a fraction of ultralight axions to dark matter of a few percent.Comment: 24 pages, 16 figures, this version: corrected typos, some comments
added, matches published versio
The Decoupled Kinematics of High- z QSO Host Galaxies and Their Ly α Halos
© 2022. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/We present a comparison of the interstellar medium traced by [C ii] (Atacama Large Millimeter/submillimeter Array), and ionized halo gas traced by Lyα (Multi Unit Spectroscopic Explorer), in and around QSO host galaxies at z ∼ 6. To date, 18 QSOs at this redshift have been studied with both MUSE and high-resolution ALMA imaging; of these, 8 objects display a Lyα halo. Using data cubes matched in velocity resolution, we compare and contrast the spatial and kinematic information of the Lyα halos and the host galaxies’ [C ii] (and dust-continuum) emission. We find that the Lyα halos extend typically 3−30 times beyond the interstellar medium of the host galaxies. The majority of the Lyα halos do not show ordered motion in their velocity fields, whereas most of the [C ii] velocity fields do. In those cases where a velocity gradient can be measured in Lyα, the kinematics do not align with those derived from the [C ii] emission. This implies that the Lyα emission is not tracing the outskirts of a large rotating disk, which is a simple extension of the central galaxy seen in [C ii] emission. It rather suggests that the kinematics of the halo gas are decoupled from those of the central galaxy. Given the scattering nature of Lyα, these results need to be confirmed with James Webb Space Telescope Integral Field Unit observations that can constrain the halo kinematics further using the nonresonant Hα line.Peer reviewe