101 research outputs found
Super-Eddington growth of the first black holes
The assembly of the first super massive black holes (SMBHs) at z ≳ 6 is still a subject of intense debate. If black holes (BHs) grow at their Eddington rate, they must start from ≳104 M⊙ seeds formed by the direct collapse of gas. Here, we explore the alternative scenario where ˜100 M⊙ BH remnants of the first stars grow at super-Eddington rate via radiatively inefficient slim accretion discs. We use an improved version of the cosmological, data-constrained semi-analytic model GAMETE/QSODUST, where we follow the evolution of nuclear BHs and gas cooling, disc and bulge formation of their host galaxies. Adopting SDSS J1148+5251 (J1148) at z = 6.4 as a prototype of luminous z ≳ 6 quasars, we find that ˜80 per cent of its SMBH mass is grown by super-Eddington accretion, which can be sustained down to z ˜ 10 in dense, gas-rich environments. The average BH mass at z ˜ 20 is MBH ≳ 104 M⊙, comparable to that of direct collapse BHs. At z = 6.4 the AGN-driven mass outflow rate is consistent with the observations and the BH-to-bulge mass ratio is compatible with the local scaling relation. However, the stellar mass in the central 2.5 kpc is closer to the value inferred from CO observations. Finally, ˜20 per cent of J1148 progenitors at z = 7.1 have BH luminosities and masses comparable to ULAS J1120+0641, suggesting that this quasar may be one of the progenitors of J1148
On the formation of the first quasars
Observations of the most luminous quasars at redshift z>6 reveal the
existence of numerous supermasssive black holes (>10^9 Msun) already in place
about twelve billion years ago. In addition, the interstellar medium of the
galaxies hosting these black holes are observed to be chemically mature
systems, with metallicities (Z>Zsun) and dust masses (>10^8 Msun) similar to
that of more evolved, local galaxies. The connection between the rapid growth
of the first supermassive black holes and the fast chemical evolution of the
host galaxy is one of the most puzzling issues for theoretical models. Here we
review state-of-the-art theoretical models that focus on this problem with
particular emphasis on the conditions that lead to the formation of quasar
seeds and their subsequent evolution at z>6
Chasing the observational signatures of seed black holes at z > 7: candidate statistics
Supermassive black holes (SMBHs) of 10^9-10^10 Msun were already in place ~13
Gyr ago, at z>6. Super-Eddington growth of low-mass BH seeds (~100 Msun) or
less extreme accretion onto ~10^5 Msun seeds have been recently considered as
the main viable routes to these SMBHs. Here we study the statistics of these
SMBH progenitors at z~6. The growth of low- and high-mass seeds and their host
galaxies are consistently followed using the cosmological data constrained
model GAMETE/QSOdust, which reproduces the observed properties of high-z
quasars, like SDSS J1148+5251. We show that both seed formation channels can be
in action over a similar redshift range, 15 < z < 18 and are found in dark
matter halos with comparable mass, ~5x10^7 Msun. However, as long as the
systems evolve in isolation (i.e. no mergers occur), noticeable differences in
their properties emerge: at z>= 10 galaxies hosting high-mass seeds have
smaller stellar mass and metallicity, the BHs accrete gas at higher rates and
star formation proceeds less efficiently than in low-mass seeds hosts. At z<10
these differences are progressively erased, as the systems experience minor or
major mergers and every trace of the BH origin gets lost.Comment: accepted for publication in MNRA
From the first stars to the first black holes
The growth of the first super massive black holes (SMBHs) at z > 6 is still a
major challenge for theoretical models. If it starts from black hole (BH)
remnants of Population III stars (light seeds with mass ~ 100 Msun) it requires
super-Eddington accretion. An alternative route is to start from heavy seeds
formed by the direct collapse of gas onto a ~ 10^5 Msun BH. Here we investigate
the relative role of light and heavy seeds as BH progenitors of the first
SMBHs. We use the cosmological, data constrained semi-analytic model
GAMETE/QSOdust to simulate several independent merger histories of z > 6
quasars. Using physically motivated prescriptions to form light and heavy seeds
in the progenitor galaxies, we find that the formation of a few heavy seeds
(between 3 and 30 in our reference model) enables the Eddington-limited growth
of SMBHs at z > 6. This conclusion depends sensitively on the interplay between
chemical, radiative and mechanical feedback effects, which easily erase the
conditions that allow the suppression of gas cooling in the low metallicity gas
(Z Jcr). We find that heavy seeds can not form if dust cooling
triggers gas fragmentation above a critical dust-to-gas mass ratio (D > Dcr).
In addition, the relative importance of light and heavy seeds depends on the
adopted mass range for light seeds, as this dramatically affects the history of
cold gas along the merger tree, by both SN and AGN-driven winds.Comment: 16 pages, 13 figures, accepted for publication in MNRA
High-redshift quasars host galaxies: is there a stellar mass crisis?
We investigate the evolutionary properties of a sample of quasars at 5<z<6.4
using the semi-analytical hierarchical model GAMETE/QSOdust. We find that the
observed properties of these quasars are well reproduced by a common formation
scenario in which stars form according to a standard IMF, via quiescent star
formation and efficient merger-driven bursts, while the central BH grows via
gas accretion and BH-BH mergers. Eventually, a strong AGN driven wind starts to
clear up the ISM of dust and gas, damping the star formation and un-obscuring
the line of sight toward the QSO. In this scenario, all the QSOs hosts have
final stellar masses in the range , a factor 3-30
larger than the upper limits allowed by the observations. We discuss
alternative scenarios to alleviate this apparent tension: the most likely
explanation resides in the large uncertainties that still affect dynamical mass
measurements in these high-z galaxies. In addition, during the transition
between the starburst-dominated and the active QSO phase, we predict that about
40% of the progenitor galaxies can be classified as Sub Millimeter Galaxies,
although their number rapidly decreases with redshift.Comment: 14 pages, 9 figures, accepted for publication in MNRA
The sustainable growth of the first black holes
Super-Eddington accretion has been suggested as a possible formation pathway
of supermassive black holes (SMBHs) 800 Myr after the Big
Bang. However, stellar feedback from BH seed progenitors and winds from BH
accretion disks may decrease BH accretion rates. In this work, we study the
impact of these physical processes on the formation of quasar,
including new physical prescriptions in the cosmological, data-constrained
semi-analytic model GAMETE/QSOdust. We find that the feedback produced by the
first stellar progenitors on the surrounding does not play a relevant role in
preventing SMBHs formation. In order to grow the SMBHs, the
accreted gas must efficiently lose angular momentum. Moreover disk winds,
easily originated in super-Eddington accretion regime, can strongly reduce duty
cycles. This produces a decrease in the active fraction among the progenitors
of bright quasars, reducing the probability to observe them
Chasing the observational signatures of seed black holes at z > 7: candidate observability
Observing the light emitted by the first accreting black holes (BHs) would
dramatically improve our understanding of the formation of quasars at z > 6,
possibly unveiling the nature of their supermassive black hole (SMBH) seeds. In
previous works we explored the relative role of the two main competing BH seed
formation channels, Population III remnants (low-mass seeds) and direct
collapse BHs (high-mass seeds), investigating the properties of their host
galaxies in a cosmological context. Building on this analysis, we predict here
the spectral energy distribution and observational features of low- and
high-mass BH seeds selected among the progenitors of a z~6 SMBH. We derive the
processed emission from both accreting BHs and stars by using the
photo-ionization code Cloudy, accounting for the evolution of metallicity and
dust-to-gas mass ratio in the interstellar medium of the host galaxies, as
predicted by the cosmological data- constrained model GAMETE/QSOdust. We show
how future missions like JWST and ATHENA will be able to detect the light
coming from SMBH progenitors already at z~16. We build upon previous
complementary studies and propose a method based on the combined analysis of
near infrared (NIR) colors, IR excess (IRX) and UV continuum slopes (i.e.
color-color and IRX-Beta diagrams) to distinguish growing seed BH host galaxies
from starburst-dominated systems in JWST surveys. Sources selected through this
criterion would be the best target for follow-up X-ray observations.Comment: accepted for publicaztion in MNRA
Chasing the observational signatures of seed black holes at z > 7: Candidate statistics
Supermassive black holes (SMBHs) of 109–1010M were already in place ∼13 Gyr ago, at
z > 6. Super-Eddington growth of low-mass BH seeds (∼100M) or less extreme accretion
on to∼105M seeds have been recently considered as the main viable routes to these SMBHs.
Here, we study the statistics of these SMBH progenitors at z ∼ 6. The growth of low- and
high-mass seeds and their host galaxies are consistently followed using the cosmological data
constrained model GAMETE/QSODUST, which reproduces the observed properties of high-z
quasars, like SDSS J1148+5251.We show that both seed formation channels can be in action
over a similar redshift range 15 < z < 18 and are found in dark matter haloes with comparable
mass, ∼5 × 107M. However, as long as the systems evolve in isolation (i.e. no mergers
occur), noticeable differences in their properties emerge: At z ≥ 10 galaxies hosting high-mass
seeds have smaller stellar mass and metallicity, the BHs accrete gas at higher rates and star
formation proceeds less efficiently than in low-mass seeds hosts. At z < 10 these differences
are progressively erased, as the systems experience minor or major mergers and every trace of
the BH origin gets lost
Faint progenitors of luminous z ∼ 6 quasars: Why do not we see them?
Observational searches for faint active nuclei at z > 6 have been extremely elusive, with a few candidates whose high-z nature is still to be confirmed. Interpreting this lack of detections is crucial to improve our understanding of high-z supermassive black holes (SMBHs) formation and growth. In this work, we present a model for the emission of accreting black holes (BHs) in the X-ray band, taking into account super-Eddington accretion, which can be very common in gas-rich systems at high-z. We compute the spectral energy distribution for a sample of active galaxies simulated in a cosmological context, which represent the progenitors of a z ˜ 6 SMBH with MBH ˜ 109 M⊙. We find an average Compton-thick fraction of ˜45 per cent and large typical column densities (NH ≳ 1023 cm2). However, faint progenitors are still luminous enough to be detected in the X-ray band of current surveys. Even accounting for a maximum obscuration effect, the number of detectable BHs is reduced at most by a factor of 2. In our simulated sample, observations of faint quasars are mainly limited by their very low active fraction (fact ˜ 1 per cent), which is the result of short, supercritical growth episodes. We suggest that to detect high-z SMBHs progenitors, large area surveys with shallower sensitivities, such as COSMOS Legacy and XMM-LSS+XXL, are to be preferred with respect to deep surveys probing smaller fields, such as Chandra Deep Field South
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