52 research outputs found
Evolution in the black hole mass-bulge mass relation: a theoretical perspective
We explore the growth of super-massive black holes and host galaxy bulges in
the galaxy population using the Millennium Run LCDM simulation coupled with a
model of galaxy formation. We find that, if galaxy mergers are the primary
drivers for both bulge and black hole growth, then in the simplest picture one
should expect the mBH-mbulge relation to evolve with redshift, with a larger
black hole mass associated with a given bulge mass at earlier times relative to
the present day. This result is independent of an evolving cold gas fraction in
the galaxy population. The evolution arises from the disruption of galactic
disks during mergers that make a larger fractional mass contribution to bulges
at low redshift than at earlier epochs. There is no comparable growth mode for
the black hole population. Thus, this effect produces evolution in the
mBH-mbulge relation that is driven by bulge mass growth and not by black holes.Comment: 6 pages, 3 figures, minor revisions, replaced with accepted MNRAS
versio
Supermassive black hole ancestors
We study a model in which supermassive black holes (SMBHs) can grow by the
combined action of gas accretion on heavy seeds and mergers of both heavy
(m_s^h=10^5 Msol) and light (m_s^l = 10^2 Msol) seeds. The former result from
the direct collapse of gas in T_s^h >1.5x10^4K, H_2-free halos; the latter are
the endproduct of a standard H_2-based star formation process. The H_2-free
condition is attained by exposing halos to a strong (J_21 > 10^3) Lyman-Werner
UV background produced by both accreting BHs and stars, thus establishing a
self-regulated growth regime. We find that this condition is met already at z
close to 18 in the highly biased regions in which quasars are born. The key
parameter allowing the formation of SMBHs by z=6-7 is the fraction of halos
that can form heavy seeds: the minimum requirement is that f_heavy>0.001; SMBH
as large as 2x10^10 Msol can be obtained when f_heavy approaches unity.
Independently of f_heavy, the model produces a high-z stellar bulge-black hole
mass relation which is steeper than the local one, implying that SMBHs formed
before their bulge was in place. The formation of heavy seeds, allowed by the
Lyman-Werner radiative feedback in the quasar-forming environment, is crucial
to achieve a fast growth of the SMBH by merger events in the early phases of
its evolution, i.e. z>7. The UV photon production is largely dominated by stars
in galaxies, i.e. black hole accretion radiation is sub-dominant.
Interestingly, we find that the final mass of light BHs and of the SMBH in the
quasar is roughly equal by z=6; by the same time only 19% of the initial baryon
content has been converted into stars. The SMBH growth is dominated at all
epochs z > 7.2 by mergers (exceeding accretion by a factor 2-50); at later
times accretion becomes by far the most important growth channel. We finally
discuss possible shortcomings of the model.Comment: 12 pages, 9 figures, 1 table, MNRAS in pres
Tidal disruption of satellite galaxies in a semi-analytic model of galaxy formation
We introduce a new physical recipe into the De Lucia and Blaizot version of the Munich semi-analytic model built upon the Millennium dark matter simulation: the tidal stripping of stellar material from satellite galaxies during mergers. To test the significance of the new physical process we apply a Monte Carlo Markov Chain parameter estimation technique constraining the model with the -band luminosity function, colours and the black hole-bulge mass relation. The differences in parameter correlations, and in the allowed regions in likelihood space, reveal the impact of the new physics on the basic ingredients of the model, such as the star-formation laws, feedback recipes and the black hole growth model. With satellite disruption in place, we get a model likelihood four times higher than in the original model, indicating that the new process seems to be favoured by observations. This is achieved mainly due to a reduction in black hole growth that produces a better agreement between the properties of central black holes and host galaxies. Compared to the best-fit model without disruption, the new model removes the excess of dwarf galaxies in the original recipe with a more modest supernova heating. The new model is now consistent with the three observational data sets used to constrain it, while significantly improving the agreement with observations for the distribution of metals in stars. Moreover, the model now follows the build up of intra-cluster light
Feedback and the Structure of Simulated Galaxies at redshift z=2
We study the properties of simulated high-redshift galaxies using
cosmological N-body/gasdynamical runs from the OverWhelmingly Large Simulations
(OWLS) project. The runs contrast several feedback implementations of varying
effectiveness: from no-feedback, to supernova-driven winds to powerful
AGN-driven outflows. These different feedback models result in large variations
in the abundance and structural properties of bright galaxies at z=2. We find
that feedback affects the baryonic mass of a galaxy much more severely than its
spin, which is on average roughly half that of its surrounding dark matter halo
in our runs. Feedback induces strong correlations between angular momentum
content and galaxy mass that leave their imprint on galaxy scaling relations
and morphologies. Encouragingly, we find that galaxy disks are common in
moderate-feedback runs, making up typically ~50% of all galaxies at the centers
of haloes with virial mass exceeding 1e11 M_sun. The size, stellar masses, and
circular speeds of simulated galaxies formed in such runs have properties that
straddle those of large star-forming disks and of compact early-type galaxies
at z=2. Once the detailed abundance and structural properties of these rare
objects are well established it may be possible to use them to gauge the
overall efficacy of feedback in the formation of high redshift galaxies.Comment: 16 pages, 12 figures. Accepted for publication in MNRAS. Minor
changes to match published versio
On the radiative efficiencies, Eddington ratios, and duty cycles of luminous high-redshift quasars
We investigate the characteristic radiative efficiency \epsilon, Eddington
ratio \lambda, and duty cycle P_0 of high-redshift active galactic nuclei
(AGN), drawing on measurements of the AGN luminosity function at z=3-6 and,
especially, on recent measurements of quasar clustering at z=3-4.5 from the
Sloan Digital Sky Survey. The free parameters of our models are \epsilon,
\lambda, and the normalization, scatter, and redshift evolution of the relation
between black hole mass \mbh and halo virial velocity V_vir. We compute the
luminosity function from the implied growth of the black hole mass function and
the quasar correlation length from the bias of the host halos. We test our
adopted formulae for the halo mass function and halo bias against measurements
from the large N-body simulation developed by the MICE collaboration. The
strong clustering of AGNs observed at z=3 and, especially, at z=4 implies that
massive black holes reside in rare, massive dark matter halos. Reproducing the
observed luminosity function then requires high efficiency \epsilon and/or low
Eddington ratio \lambda, with a lower limit (based on 2\sigma agreement with
the measured z=4 correlation length) \epsilon> 0.7\lambda/(1+0.7\lambda),
implying \epsilon > 0.17 for \lambda > 0.25. Successful models predict high
duty cycles, P_0~0.2, 0.5, and 0.9 at z=3.1, 4.5 and 6, respectively, and they
require that the fraction of halo baryons locked in the central black hole is
much larger than the locally observed value. The rapid drop in the abundance of
the massive and rare host halos at z>7 implies a proportionally rapid decline
in the number density of luminous quasars, much stronger than simple
extrapolations of the z=3-6 luminosity function would predict. (abridged)Comment: Replaced with version accepted by ApJ. More detailed analysis
including black hole mergers. Results unchange
Modeling the cosmological co-evolution of supermassive black holes and galaxies: II. The clustering of quasars and their dark environment
We use semi-analytic modeling on top of the Millennium simulation to study
the joint formation of galaxies and their embedded supermassive black holes.
Our goal is to test scenarios in which black hole accretion and quasar
activity are triggered by galaxy mergers, and to constrain different models for
the lightcurves associated with individual quasar events. In the present work
we focus on studying the spatial distribution of simulated quasars. At all
luminosities, we find that the simulated quasar two-point correlation function
is fit well by a single power-law in the range 0.5 < r < 20 h^{-1} Mpc, but its
normalization is a strong function of redshift. When we select only quasars
with luminosities within the range typically accessible by today's quasar
surveys, their clustering strength depends only weakly on luminosity, in
agreement with observations. This holds independently of the assumed lightcurve
model, since bright quasars are black holes accreting close to the Eddington
limit, and are hosted by dark matter haloes with a narrow mass range of a few
10^12 h^{-1} M_sun. Therefore the clustering of bright quasars cannot be used
to disentangle lightcurve models, but such a discrimination would become
possible if the observational samples can be pushed to significantly fainter
limits.
Overall, our clustering results for the simulated quasar population agree
rather well with observations, lending support to the conjecture that galaxy
mergers could be the main physical process responsible for triggering black
hole accretion and quasar activity.Comment: 17 pages, 16 figures, to be published on MNRA
GECO: Galaxy Evolution COde - A new semi-analytical model of galaxy formation
We present a new semi-analytical model of galaxy formation, GECO (Galaxy
Evolution COde), aimed at a better understanding of when and how the two
processes of star formation and galaxy assembly have taken place. Our model is
structured into a Monte Carlo algorithm based on the Extended Press-Schechter
theory, for the representation of the merging hierarchy of dark matter halos,
and a set of analytic algorithms for the treatment of the baryonic physics,
including classical recipes for the gas cooling, the star formation
time-scales, galaxy mergers and SN feedback. Together with the galaxies, the
parallel growth of BHs is followed in time and their feedback on the hosting
galaxies is modelled. We set the model free parameters by matching with data on
local stellar mass functions and the BH-bulge relation at z=0. Based on such
local boundary conditions, we investigate how data on the high-redshift
universe constrain our understanding of the physical processes driving the
evolution, focusing in particular on the assembly of stellar mass and on the
star formation history. Since both processes are currently strongly constrained
by cosmological near- and far-IR surveys, the basic physics of the Lambda CDM
hierarchical clustering concept of galaxy formation can be effectively tested
by us by comparison with the most reliable set of observables. Our
investigation shows that when the time-scales of the stellar formation and mass
assembly are studied as a function of dark matter halo mass and the single
galaxy stellar mass, the 'downsizing' fashion of star formation appears to be a
natural outcome of the model, reproduced even in the absence of the AGN
feedback. On the contrary, the stellar mass assembly history turns out to
follow a more standard hierarchical pattern progressive in cosmic time, with
the more massive systems assembled at late times mainly through dissipationless
mergers.Comment: Accepted for publication in A&A, 24 pages, 15 figure
Binary Quasars at High Redshift I: 24 New Quasar Pairs at z ~ 3-4
The clustering of quasars on small scales yields fundamental constraints on
models of quasar evolution and the buildup of supermassive black holes. This
paper describes the first systematic survey to discover high redshift binary
quasars. Using color-selection and photometric redshift techniques, we searched
8142 deg^2 of SDSS imaging data for binary quasar candidates, and confirmed
them with follow-up spectroscopy. Our sample of 27 high redshift binaries (24
of them new discoveries) at redshifts 2.9 < z < 4.3 with proper transverse
separations 10 kpc < R_{\perp} < 650 kpc increases the number of such objects
known by an order of magnitude. Eight members of this sample are very close
pairs with R_{\perp} 3.5.
The completeness and efficiency of our well-defined selection algorithm are
quantified using simulated photometry and we find that our sample is ~ 50%
complete. Our companion paper uses this knowledge to make the first measurement
of the small scale clustering (R < 1 Mpc/h comoving) of high-redshift quasars.
High redshift binaries constitute exponentially rare coincidences of two
extreme (M >~ 10^9 Msun) supermassive black holes. At z ~ 4 there is about one
close binary per 10 Gpc^3, thus these could be the highest sigma peaks, the
analogs of superclusters, in the early Universe.Comment: Submitted to Ap
The evolution of massive black holes and their spins in their galactic hosts
[Abridged] [...] We study the mass and spin evolution of massive black holes
within a semianalytical galaxy-formation model that follows the evolution of
dark-matter halos along merger trees, as well as that of the baryonic
components (hot gas, stellar and gaseous bulges, and stellar and gaseous
galactic disks). This allows us to study the mass and spin evolution of massive
black holes in a self-consistent way, by taking into account the effect of the
gas present in galactic nuclei both during the accretion phases and during
mergers. Also, we present predictions, as a function of redshift, for the
fraction of gas-rich black-hole mergers -- in which the spins prior to the
merger are aligned due to the gravito-magnetic torques exerted by the
circumbinary disk -- as opposed to gas-poor mergers, in which the orientation
of the spins before the merger is roughly isotropic. These predictions may be
tested by LISA or similar spaced-based gravitational-wave detectors such as
eLISA/NGO or SGO.Comment: 26 pages, 15 figures. This version includes minor changes to figs 10
and 11 (left-hand panels) described in erratum (MNRAS 440, 1295, 2014, doi:
10.1093/mnras/stu361), see also http://www2.iap.fr/users/barausse/erratum.pd
The Demography of Super-Massive Black Holes: Growing Monsters at the Heart of Galaxies
Supermassive black holes (BHs) appear to be ubiquitous at the center of all
galaxies which have been observed at high enough sensitivities and resolution
with the Hubble Space Telescope. Their masses are found to be tightly linked
with the masses and velocity dispersions of their host galaxies. On the other
hand, BHs are widely held to constitute the central engines of quasars and
active galactic nuclei (AGN) in general. It is however still unclear how BHs
have grown, and whether they have co-evolved with their hosts. In this Review I
discuss how, in ways independent of specific models, constraints on the growth
history of BHs and their host galaxies have been set by matching the statistics
of local BHs to the emissivity, number density, and clustering properties of
AGNs at different cosmological epochs. I also present some new results obtained
through a novel numerical code which evolves the BH mass function and
clustering adopting broad distributions of Eddington ratios. I finally review
BH evolution in a wider cosmological context, connecting BH growth to galaxy
evolution.Comment: 70 pages. New Astronomy Reviews, in pres
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