33 research outputs found
Clustering of X-Ray-Selected AGN
The study of the angular and spatial structure of the X-ray sky has been
under investigation since the times of the Einstein X-ray Observatory. This
topic has fascinated more than two generations of scientists and slowly
unveiled an unexpected scenario regarding the consequences of the angular and
spatial distribution of X-ray sources. It was first established from the
clustering of sources making the CXB that the source spatial distribution
resembles that of optical QSO. It then it became evident that the distribution
of X-ray AGN in the Universe was strongly reflecting that of Dark Matter. In
particular one of the key result is that X-ray AGN are hosted by Dark Matter
Halos of mass similar to that of galaxy groups. This result, together with
model predictions, has lead to the hypothesis that galaxy mergers may
constitute the main AGN triggering mechanism. However detailed analysis of
observational data, acquired with modern telescopes, and the use of the new
Halo Occupation formalism has revealed that the triggering of an AGN could also
be attributed to phenomena like tidal disruption or disk instability, and to
galaxy evolution. This paper reviews results from 1988 to 2011 in the field of
X-ray selected AGN clustering.Comment: 19 pages, 4 Figures, review paper published on Advances in Astronomy
Special Issue "Seeking for the Leading Actor on the Cosmic Stage: Galaxies
versus Supermassive Black Holes", v2, final versio
Clustering of gamma-ray selected 2LAC Fermi Blazars
We present the first measurement of the projected correlation function of 485
gamma-ray selected Blazars, divided in 175 BLLacertae (BL Lacs) and 310 Flat
Spectrum Radio Quasars (FSRQs) detected in the 2-year all-sky survey by
Fermi-Large Area Telescope. We find that Fermi BL Lacs and FSRQs reside in
massive dark matter halos (DMHs) with logMh=13.35+0.20/-0.14 and logMh =
13.40+0.15/-0.19 Msun/h, respectively, at low (z=0.4) and high (z =1.2)
redshift. In terms of clustering properties, these results suggest that BL Lacs
and FSRQs are similar objects residing in the same dense environment typical of
galaxy groups, despite their different spectral energy distribution, power and
accretion rate. We find no difference in the typical bias and hosting halo mass
between Fermi Blazars and radio-loud AGN, supporting the unifcation scheme
simply equating radio-loud objects with misaligned Blazar counterparts. This
similarity in terms of typical environment they preferentially live in,
suggests that Blazars preferentially occupy the centre of DMHs, as already
pointed out for radio-loud AGN. This implies, in light of several projects
looking for the gamma-ray emission from DM annihilation in galaxy clusters, a
strong contamination from Blazars to the expected signal from DM annihilation.Comment: Accepted for publication in The Astrophysical Journa
X-ray variability with WFXT: AGNs, transients and more
The Wide Field X-ray Telescope (WFXT) is a proposed mission with a high
survey speed, due to the combination of large field of view (FOV) and effective
area, i.e. grasp, and sharp PSF across the whole FOV. These characteristics
make it suitable to detect a large number of variable and transient X-ray
sources during its operating lifetime. Here we present estimates of the WFXT
capabilities in the time domain, allowing to study the variability of thousand
of AGNs with significant detail, as well as to constrain the rates and
properties of hundreds of distant, faint and/or rare objects such as X-ray
Flashes/faint GRBs, Tidal Disruption Events, ULXs, Type-I bursts etc. The
planned WFXT extragalactic surveys will thus allow to trace variable and
transient X-ray populations over large cosmological volumes.Comment: Proceedings of "The Wide Field X-ray Telescope Workshop", held in
Bologna, Italy, Nov. 25-26 2009 (arXiv:1010.5889). To appear in Memorie della
Societ\`a Astronomica Italiana 2010 - Minor corrections to text
Large-scale clustering of buried X-ray AGN: Trends in AGN obscuration and redshift evolution
In order to test active galactic nucleus (AGN) unification and evolutionary
models, we measured the AGN clustering properties as a function of AGN
obscuration defined in terms of hydrogen column density, . In
addition to measuring the clustering of unobscured () and moderately obscured () AGNs,
we also targeted highly obscured sources () up to
redshifts of . We have compiled one of the largest samples of
X-ray-selected AGNs from a total of eight deep XMM/Chandra surveys. We measured
the clustering as a function of both AGN obscuration and redshift using the
projected two-point correlation function, . We modeled
the large-scale clustering signal, measured the AGN bias, ,
and interpreted it in terms of the typical AGN host dark matter halo, ). We find no significant dependence of AGN clustering on
obscuration, suggesting similar typical masses of the hosting halos as a
function of . This result matches expectations of AGN unification
models, in which AGN obscuration depends mainly on the viewing angle of the
obscuring torus. We measured, for the first time, the clustering of highly
obscured AGNs and find that these objects reside in halos with typical mass
() at low (high ) redshifts. We
find that irrespective of obscuration, an increase in AGN bias with redshift is
slower than the expectation for a constant halo mass and instead follows the
growth rate of halos, known as the passive evolution track. This implies that
for those AGNs the clustering is mainly driven by the mass growth rate of the
hosting halos and galaxies across cosmic time.Comment: 13 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Are there more galaxies than we see around high- quasars?
Whether or not quasars lie in the most massive dark-matter
halos of the Universe is still a subject of dispute. While most theoretical
studies support this scenario, current observations yield discordant results
when they probe the halo mass through the detection rate of quasar companion
galaxies. Feedback processes from supermassive black holes and dust obscuration
have been blamed for this discrepancy, but the impact of these effects is
complex and far from being clearly understood. This paper aims to improve the
interpretation of current far-infrared observations by taking into account the
cosmological volume probed by the Atacama Large Millimeter/submillimeter Array
Telescope and to explain the observational discrepancies. We statistically
investigate the detection rate of quasar companions in current observations and
verify if they match the expected distribution from various theoretical models,
once convolved with the ALMA field-of-view, through the use of Monte Carlo
simulations. We demonstrate that the telescope geometrical bias is fundamental
and can alone explain the scatter in the number of detected satellite galaxies
in different observations. We conclude that the resulting companion densities
depend on the chosen galaxy distributions. According to our fiducial models,
current data favour a density scenario where quasars lie in dark-matter halos
of viral mass , in agreement with
most theoretical studies. According to our analysis, each quasar has about 2
companion galaxies, with a [CII] luminosity , within a distance of about 1~Mpc from the quasar.Comment: 7 pages, 5 figures; accepted for publications in A&
Black hole scaling relations of active and quiescent galaxies: Addressing selection effects and constraining virial factors
Local samples of quiescent galaxies with dynamically measured black hole
masses (Mbh) may suffer from an angular resolution-related selection effect,
which could bias the observed scaling relations between Mbh and host galaxy
properties away from the intrinsic relations. In particular, previous work has
shown that the observed Mbh-Mstar (stellar mass) relation is more strongly
biased than the Mbh-sigma (velocity dispersion) relation. Local samples of
active galactic nuclei (AGN) do not suffer from this selection effect, as in
these samples Mbh is estimated from megamasers and/or reverberation
mapping-based techniques. With the exception of megamasers, Mbh-estimates in
these AGN samples are proportional to a virial coefficient fvir. Direct
modelling of the broad line region suggests that fvir~3.5. However, this
results in a Mbh-Mstar relation for AGN which lies below and is steeper than
the one observed for quiescent black hole samples. A similar though milder
trend is seen for the Mbh-sigma relation. Matching the high-mass end of the
Mbh-Mstar and Mbh-sigma relations observed in quiescent samples requires
fvir~15 and fvir~7, respectively. On the other hand, fvir~3.5 yields Mbh-sigma
and Mbh-Mstar relations for AGN which are remarkably consistent with the
expected `intrinsic' correlations for quiescent samples (i.e., once account has
been made of the angular resolution-related selection effect), providing
additional evidence that the sample of local quiescent black holes is biased.
We also show that, as is the case for quiescent black holes, the Mbh-Mstar
scaling relation of AGN is driven by velocity dispersion, thus providing
additional key constraints to black hole-galaxy co-evolution models.Comment: 15 pages, 5 Figures. MNRAS, accepte
Selection bias in dynamically measured supermassive black hole samples : its consequences and the quest for the most fundamental relation
We compare the set of local galaxies having dynamically measured black holes with a large, unbiased sample of galaxies extracted from the Sloan Digital Sky Survey. We confirm earlier work showing that the majority of black hole hosts have significantly higher velocity dispersions sigma than local galaxies of similar stellar mass. We use Monte Carlo simulations to illustrate the effect on black hole scaling relations if this bias arises from the requirement that the black hole sphere of influence must be resolved to measure black hole masses with spatially resolved kinematics. We find that this selection effect artificially increases the normalization of the M-bh-sigma relation by a factor of at least similar to 3; the bias for the M-bh-M-star relation is even larger. Our Monte Carlo simulations and analysis of the residuals from scaling relations both indicate that sigma is more fundamental than M-star or effective radius. In particular, the M-bh-M-star relation is mostly a consequence of the M-bh-sigma and sigma-M-star relations, and is heavily biased by up to a factor of 50 at small masses. This helps resolve the discrepancy between dynamically based black hole-galaxy scaling relations versus those of active galaxies. Our simulations also disfavour broad distributions of black hole masses at fixed sigma. Correcting for this bias suggests that the calibration factor used to estimate black hole masses in active galaxies should be reduced to values of f(vir) similar to 1. Black hole mass densities should also be proportionally smaller, perhaps implying significantly higher radiative efficiencies/black hole spins. Reducing black hole masses also reduces the gravitational wave signal expected from black hole mergers.Peer reviewe
The Black Hole Mass Function across Cosmic Time. II. Heavy Seeds and (Super)Massive Black Holes
This is the second paper in a series aimed at modeling the black hole (BH) mass function from the stellar to the (super)massive regime. In the present work, we focus on (super)massive BHs and provide an ab initio computation of their mass function across cosmic time. We consider two main mechanisms to grow the central BH that are expected to cooperate in the high-redshift star-forming progenitors of local massive galaxies. The first is the gaseous dynamical friction process, which can cause the migration toward the nuclear regions of stellar mass BHs originated during the intense bursts of star formation in the gas-rich host progenitor galaxy and the buildup of a central heavy BH seed, M• ∼ 103−5 M⊙, within short timescales of ≲some 107 yr. The second mechanism is the standard Eddington-type gas disk accretion onto the heavy BH seed through which the central BH can become (super)massive, M• ∼ 106−10 M⊙, within the typical star formation duration, ≲1 Gyr, of the host. We validate our semiempirical approach by reproducing the observed redshift-dependent bolometric AGN luminosity functions and Eddington ratio distributions and the relationship between the star formation and the bolometric luminosity of the accreting central BH. We then derive the relic (super)massive BH mass function at different redshifts via a generalized continuity equation approach and compare it with present observational estimates. Finally, we reconstruct the overall BH mass function from the stellar to the (super)massive regime over more than 10 orders of magnitudes in BH mass