Abstract
Motivated by evidences favoring a rapid and late hydrogen reionization process completing at
z
∼ 5.2–5.5 and mainly driven by rare and luminous sources, we have reassessed the estimate of ...the space density of ultra-luminous QSOs at
z
∼ 5 in the framework of the QUBRICS survey. A ∼ 90% complete sample of 14 spectroscopically confirmed QSOs at
M
1450
≤ −28.3 and 4.5 ≤
z
≤ 5.0 has been derived in an area of 12,400 deg
2
, thanks to multiwavelength selection and Gaia astrometry. The space density of
z
∼ 5 QSOs within −29.3 ≤
M
1450
≤ −28.3 is three times higher than previous determinations. Our results suggest a steep bright-end slope for the QSO luminosity function at
z
∼ 5 and a mild redshift evolution of the space density of ultrabright QSOs (
M
1450
∼ −28.5) at 3 <
z
< 5.5, in agreement with the redshift evolution of the much fainter active galactic nucleus (AGN) population at
M
1450
∼ −23. These findings are consistent with a pure density evolution for the AGN population at
z
> 3. Adopting our
z
∼ 4 QSO luminosity function and applying a mild density evolution in redshift, a photoionization rate of
Γ
HI
=
0.46
−
0.09
+
0.17
×
10
−
12
s
−
1
has been obtained at
z
= 4.75, assuming an escape fraction of ∼70% and a steep faint-end slope of the AGN luminosity function. The derived photoionization rate is ∼50–100% of the ionizing background measured at the end of the reionization epoch, suggesting that AGNs could play an important role in the cosmological reionization process.
Recent inspections of local available data suggest that the almost linear relation between the stellar mass of spheroids (M
sph) and the mass of the super massive black holes (BHs), residing at their ...centres, shows a break below M
sph ∼ 1010 M⊙, with a steeper, about quadratic relation at smaller masses. We investigate the physical mechanisms responsible for the change in slope of this relation, by comparing data with the results of the semi-analytic model of galaxy formation morgana, which already predicted such a break in its original formulation. We find that the change of slope is mostly induced by effective stellar feedback in star-forming bulges. The shape of the relation is instead quite insensitive to other physical mechanisms connected to BH accretion such as disc instabilities, galaxy mergers, active galactic nucleus (AGN) feedback, or even the exact modelling of accretion on to the BH, direct or through a reservoir of low angular momentum gas. Our results support a scenario where most stars form in the disc component of galaxies and are carried to bulges through mergers and disc instabilities, while accretion on to BHs is connected to star formation in the spheroidal component. Therefore, a model of stellar feedback that produces stronger outflows in star-forming bulges than in discs will naturally produce a break in the scaling relation. Our results point to a form of co-evolution especially at lower masses, below the putative break, mainly driven by stellar feedback rather than AGN feedback.
ABSTRACT
We extend our previous work focused at z ∼ 0, studying the redshift evolution of galaxy dynamical properties using the state-of-the-art semi-analytic model GAEA (GAlaxy Evolution and ...Assembly): we show that the predicted size–mass relation for discy/star-forming and quiescent galaxies is in good agreement with observational estimates, up to z ∼ 2. Bulge-dominated galaxies have sizes that are offset low with respect to observational estimates, mainly due to our implementation of disc instability at high redshift. At large masses, both quiescent and bulge-dominated galaxies have sizes smaller than observed. We interpret this as a consequence of our most massive galaxies having larger gas masses than observed, and therefore being more affected by dissipation. We argue that a proper treatment of quasar-driven winds is needed to alleviate this problem. Our model compact galaxies have number densities in agreement with observational estimates and they form most of their stars in small and low angular momentum high-z haloes. GAEA predicts that a significant fraction of compact galaxies forming at high-z is bound to merge with larger structures at lower redshifts: therefore they are not the progenitors of normal-size passive galaxies at z = 0. Our model also predicts a stellar–halo size relation that is in good agreement with observational estimates. The ratio between stellar size and halo size is proportional to the halo spin and does not depend on stellar mass but for the most massive galaxies, where active galactic nucleus feedback leads to a significant decrease of the retention factor (from about 80 per cent to 20 per cent).
Abstract
Based on results by recent surveys, the number of bright quasars at redshifts
z
> 3 is being constantly revised upward. The current consensus is that at bright magnitudes (
M
1450
≤ −27) the ...number densities of such sources could have been underestimated by a factor of 30%–40%. In the framework of the QUBRICS survey, we identified 58 bright QSOs at 3.6 ≤
z
≤ 4.2, with magnitudes
i
psf
≤ 18, in an area of 12400 deg
2
. The uniqueness of our survey is underlined by the fact that it allows us, for the first time, to extend the sampled absolute magnitude range up to
M
1450
= −29.5. We derived a bright-end slope of
β
= −4.025 and a space density at 〈
M
1450
〉 = −28.75 of 2.61 × 10
−10
Mpc
−3
comoving, after taking into account the estimated incompleteness of our observations. Taking into account the results of fainter surveys, active galactic nuclei (AGNs) brighter than
M
1450
= −23 could produce at least half of the ionizing emissivity at
z
∼ 4. Considering a mean escape fraction of 0.7 for the QSO and AGN population, combined with a mean free path of 41.3 proper Mpc at
z
= 3.9, we derive a photoionization rate of
, produced by AGNs at
M
1450
< −18, that is, ∼100% of the measured ionizing background at
z
∼ 4.
Galaxies grow primarily via accretion-driven star formation in discs and merger-driven growth of bulges. These processes are implicit in semi-analytical models of galaxy formation, with bulge growth ...in particular relating directly to the hierarchical build-up of haloes and their galaxies. In this paper, we consider several implementations of two semi-analytical models. Focusing on implementations in which bulges are formed during mergers only, we examine the fractions of elliptical galaxies and both passive and star-forming disc galaxies as functions of stellar and halo mass, for central and satellite systems. This is compared to an observational cross-matched Sloan Digital Sky Survey+Third Reference Catalog of Bright Galaxies z ∼ 0 sample of galaxies with accurate visual morphological classifications and M
* > 1010.5 M. The models qualitatively reproduce the observed increase of elliptical fraction with stellar mass, and with halo mass for central galaxies, supporting the idea that observed ellipticals form during major mergers. However, the overall elliptical fraction produced by the models is much too high compared with the z ∼ 0 data. Since the 'passive' - i.e. non-star-forming - fractions are approximately reproduced, and since the fraction which are star-forming disc galaxies is also reproduced, the problem is that the models overproduce ellipticals at the expense of passive S0 and spiral galaxies. Bulge growth implementations (tuned to reproduce simulations) which allow the survival of residual discs in major mergers still destroy too much of the disc. Increasing the lifetime of satellites, or allowing significant disc regrowth around merger remnants, merely increases the fraction of star-forming disc galaxies. Instead, it seems necessary to reduce the mass ratios of merging galaxies, so that most mergers produce modest bulge growth in disc galaxy remnants instead of ellipticals. This could be a natural consequence of tidal stripping of stars from infalling satellite galaxies, a process not considered in our models. However, a high efficiency of quenching during and/or subsequent to minor mergers is still required to keep the passive fraction high.
Modifications of the equations of general relativity at large distances offer one possibility to explain the observed properties of our Universe without invoking a cosmological constant. Numerous ...proposals for such modified gravity cosmologies exist, but often their consequences for structure formation in the non-linear sector are not yet accurately known. In this work, we employ high-resolution numerical simulations of f (R)-gravity models coupled with a semi-analytic model (SAM) for galaxy formation to obtain detailed predictions for the evolution of galaxy properties. The f (R)-gravity models imply the existence of a 'fifth-force', which is however locally suppressed, preserving the successes of general relativity on Solar system scales. We show that dark matter haloes in f (R)-gravity models are characterized by a modified virial scaling with respect to the Λ cold dark matter (ΛCDM ) scenario, reflecting a higher dark matter velocity dispersion at a given mass. This effect is taken into account in the SAM by an appropriate modification of the mass-temperature relation. We find that the statistical properties predicted for galaxies (such as the stellar mass function and the cosmic star formation rate) in f (R)-gravity show generally only very small differences relative to ΛCDM, smaller than the dispersion between the results of different SAM models, which can be viewed as a measure of their systematic uncertainty. We also demonstrate that galaxy bias is not able to disentangle between f (R)-gravity and the standard cosmological scenario. However, f (R)-gravity imprints modifications in the linear growth rate of cosmic structures at large scale, which can be recovered from the statistical properties of large galaxy samples.
Recent studies proposed that cosmic rays (CR) are a key ingredient in setting the conditions for star formation, thanks to their ability to alter the thermal and chemical state of dense gas in the ...UV-shielded cores of molecular clouds. In this paper, we explore their role as regulators of the stellar initial mass function (IMF) variations, using the semi-analytic model for GAlaxy Evolution and Assembly (GAEA). The new model confirms our previous results obtained using the integrated galaxy-wide IMF (IGIMF) theory: both variable IMF models reproduce the observed increase of α-enhancement as a function of stellar mass and the measured z = 0 excess of dynamical mass-to-light ratios with respect to photometric estimates assuming a universal IMF. We focus here on the mismatch between the photometrically-derived (M app ⋆) and intrinsic (M ⋆) stellar masses, by analysing in detail the evolution of model galaxies with different values of M ⋆ /M app ⋆. We find that galaxies with small deviations (i.e. formally consistent with a universal IMF hypothesis) are characterized by more extended star formation histories and live in less massive haloes with respect to the bulk of the galaxy population. While the IGIMF theory does not change significantly the mean evolution of model galaxies with respect to the reference model, a CR-regulated IMF implies shorter star formation histories and higher peaks of star formation for objects more massive than 10 10.5 M ⊙. However, we also show that it is difficult to unveil this behaviour from observations, as the key physical quantities are typically derived assuming a universal IMF.
Crossing the Rubicon of Reionization with z ∼ 5 QSOs Grazian, Andrea; Boutsia, Konstantina; Giallongo, Emanuele ...
Astrophysical journal/The Astrophysical journal,
09/2023, Letnik:
955, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
One of the key open questions in cosmology is the nature of sources that completed cosmological hydrogen reionization at
z
∼ 5.2. High-
z
primeval galaxies have been long considered the main ...drivers of reionization, with a minor role played by high-
z
active galactic nuclei (AGN). However, in order to confirm this scenario, it is fundamental to measure the photoionization rate produced by active SMBHs close to the Epoch of Reionization. Given the pivotal role played by spectroscopically complete observations of high-
z
QSOs, in this paper we present the first results of the RUBICON (Reionizing the Universe with Bright Cosmological Nuclei) survey. It consists of a color-selected sample of bona fide
z
∼ 5 QSO candidates from the Hyper Suprime-Cam Subaru Strategic Survey. Our QSO candidates have been validated both by photometric redshifts based on spectral energy distribution fitting and by spectroscopic redshifts, confirming that they lie at 4.5 <
z
spec
< 5.2. A relatively high space density of QSOs (Φ ∼ 1.4 × 10
−8
c Mpc
−3
) is thus confirmed at
z
∼ 5 and
M
1450
∼ −27, consistent with a pure density evolution of the AGN luminosity function from
z
= 4 to
z
= 5, with a mild density evolution rate of 0.25 dex. This indicates that AGN could play a nonnegligible role in cosmic reionization. The Rubicon of reionization has been crossed.
ABSTRACT
We present a new implementation of the GAlaxy Evolution and Assembly (gaea) semi-analytic model, that features an improved modelling of the process of cold gas accretion on to supermassive ...black hole (SMBHs), derived from both analytic arguments and high-resolution simulations. We consider different scenarios for the loss of angular momentum required for the available cold gas to be accreted on to the central SMBHs, and we compare different combinations of triggering mechanisms, including galaxy mergers and disc instabilities in star-forming discs. We compare our predictions with the luminosity function (LF) observed for active galactic nuclei (AGNs) and we confirm that a non-instantaneous accretion time-scale (either in the form of a low-angular momentum reservoir or as an assumed light-curve evolution) is needed in order to reproduce the measured evolution of the AGN-LF and the so-called AGN-downsizing trend. Moreover, we also study the impact of AGN feedback, in the form of AGN-driven outflows, on the SF properties of model galaxies, using prescriptions derived both from empirical studies and from numerical experiments. We show that AGN-driven outflows are effective in suppressing the residual star formation rate in massive galaxies (>1011 M⊙) without changing their overall assembly history. These winds also affect the SFR of lower mass galaxies, resulting in a too large fraction of passive galaxies at <1010 M⊙. Finally, we study the Eddington ratio distribution as a function of SMBH mass, showing that only objects more massive than 108 M⊙ are already in a self-regulated state as inferred from observations.
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