325 research outputs found
Black Hole Demography: From scaling relations to models
In this contributed paper I review our current knowledge of the local Black
Hole (BH) scaling relations, and their impact on the determination of the local
BH mass function. I particularly emphasize the remaining systematic
uncertainties impinging upon a secure determination of the BH mass function and
how progress can be made. I then review and discuss the evidence for a
different time evolution for separate BH-galaxy scaling relations, and how
these independent empirical evidences can be reconciled with the overall
evolution of the structural properties of the host galaxies. I conclude
discussing BH demography in the context of semi-empirical continuity accretion
models, as well as more complex evolutionary models, emphasizing the general
constraints we can set on them.Comment: 20 pages, 5 figures. Invited article for the focus issue on
astrophysical black holes in Classical and Quantum Gravity, guest editors:
D.Merritt and L. Rezzoll
Setting firmer constraints on the evolution of the most massive, central galaxies from their local abundances and ages
There is still much debate surrounding how the most massive, central galaxies
in the local universe have assembled their stellar mass, especially the
relative roles of in-situ growth versus later accretion via mergers. In this
paper, we set firmer constraints on the evolutionary pathways of the most
massive central galaxies by making use of empirical estimates on their
abundances and stellar ages. The most recent abundance matching and direct
measurements strongly favour that a substantial fraction of massive galaxies
with Mstar>3x10^11 Msun reside at the centre of clusters with mass
Mhalo>3x10^13 Msun. Spectral analysis supports ages >10 Gyrs, corresponding to
a formation redshift z_form >2. We combine these two pieces of
observationally-based evidence with the mass accretion history of their host
dark matter haloes. We find that in these massive haloes, the stellar mass
locked up in the central galaxy is comparable to, if not greater than, the
total baryonic mass at z_form. These findings indicate that either only a
relatively minor fraction of their present-day stellar mass was formed in-situ
at z_form, or that these massive, central galaxies form in the extreme scenario
where almost all of the baryons in the progenitor halo are converted into
stars. Interestingly, the latter scenario would not allow for any substantial
size growth since the galaxy's formation epoch either via mergers or expansion.
We show our results hold irrespective of systematic uncertainties in stellar
mass, abundances, galaxy merger rates, stellar initial mass function, star
formation rate and dark matter accretion histories.Comment: 11 pages, 8 figures, MNRAS, accepte
A matter of measurement: rotation velocities and the velocity function of dwarf galaxies
The velocity function derived from large scale surveys can be compared with
the predictions of LCDM cosmology, by matching the measured rotation velocities
Vrot of galaxies to the maximum circular velocity of dark matter (DM) halos
Vmax. For Vrot<50km/s, a major discrepancy arises between the observed and LCDM
velocity functions. However, the manner in which different observational
measures of Vrot are associated with Vmax is not straight forward in dwarf
galaxies. We instead relate galaxies to DM halos using the empirical baryon-
mass to halo-mass relation, and show that different observational measures of
Vrot result in very different velocity functions. We show how the W50 velocity
function, i.e. using the HI profile line width at 50% of peak HI flux to
measure Vrot, can be reconciled with a LCDM cosmology. Our semi-empirical
methodology allows us to determine the region of rotation curves that are
probed by HI measurements (RHI), and shows that the Vrot of dwarfs are
generally measured at a fraction of Rmax, explaining their tendency to have
rising rotation curves. We provide fitting formulae for relating RHI and Reff
(the effective radius) to the virial radius of DM halos. To continue to use
velocity functions as a probe of LCDM cosmology, it is necessary to be precise
about how the different measures of rotation velocity are probing the mass of
the DM halos, dropping the assumption that any measure of rotational velocity
can be equally used as a proxy for Vmax.Comment: submitted to MNRAS. Comments very welcom
Interpreting the possible break in the Black Hole - Bulge mass relation
Recent inspections of local available data suggest that the almost linear
relation between the stellar mass of spheroids () and the mass of
the super massive Black Holes (BHs) residing at their centres, shows a break
below , 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 onto 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 onto 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.Comment: MNRAS accepted, 10 pages, 6 figures, 1 tabl
Super-massive Black Holes: the missing link in galaxy evolution
The aim of this thesis is to investigate the key points concerning the evolution of the local Super-massive Black Hole (SMBH) population and to understand the origin of the links the SMBH have with their host spheroidal galaxies and dark matter (DM) halos. In fact the tight correlations observed among the SMBH mass, the photometric and dynamical properties of the host galaxy and DM halo, suggest that these systems must have co-evolved during time building up together.
The study of the statistical mass distribution of the local SMBH population and galaxies and their link with the overall AGN statistics are fundamental tools to get important physical insights on the basic average features of SMBH evolution.
Any galaxy formation model must compare with such findings. For this purpose I have worked with the semi-analytical code developed by Granato et al. (2004) to physically model SMBH evolution in galaxies and DM halos, compare model results with empirical derivations and make predictions. In particular, my work has been primarily to tune the parameters and also try to arrange minor modifications, where needed, in order to cope with the variety of the numerous empirical data the model can successfully account for
Is there a "too big to fail" problem in the field?
We use the Arecibo legacy fast ALFA (ALFALFA) 21cm survey to measure the
number density of galaxies as a function of their rotational velocity,
(as inferred from the width of their 21cm emission line).
Based on the measured velocity function we statistically connect galaxies with
their host halo, via abundance matching. In a lambda cold dark matter
(CDM) cosmology, dwarf galaxies are expected to be hosted by halos
that are significantly more massive than indicated by the measured galactic
velocity; if smaller halos were allowed to host galaxies, then ALFALFA would
measure a much higher galactic number density. We then seek observational
verification of this predicted trend by analyzing the kinematics of a
literature sample of gas-rich dwarf galaxies. We find that galaxies with
are
kinematically incompatible with their predicted CDM host halos, in the
sense that hosts are too massive to be accommodated within the measured
galactic rotation curves. This issue is analogous to the "too big to fail"
problem faced by the bright satellites of the Milky Way, but here it concerns
extreme dwarf galaxies in the field. Consequently, solutions based on
satellite-specific processes are not applicable in this context. Our result
confirms the findings of previous studies based on optical survey data and
addresses a number of observational systematics present in these works.
Furthermore, we point out the assumptions and uncertainties that could strongly
affect our conclusions. We show that the two most important among them -namely
baryonic effects on the abundances of halos and on the rotation curves of
halos- do not seem capable of resolving the reported discrepancy.Comment: v3 matches the version published in A&A. Main differences with v2 are
in Secs 3.2 & 4.4 and the addition of Appendix B. 11 figures, 14 pages (+2
appendices
2MASS Reveals a Large Intrinsic Fraction of BALQSOs
The intrinsic fraction of broad absorption line quasars (BALQSOs) is
important in constraining geometric and evolutionary models of quasars. We
present the fraction of BALQSOs in 2MASS detected quasars within the SDSS DR3
sample in the redshift range of 1.7 < z < 4.38. The fraction of BALQSOs is
40.4^{+3.4}_{-3.3}% in the 2MASS 99% database K_s band completeness sample, and
38.5^{+1.7}_{-1.7}% in the larger 2MASS sample extending below the completeness
limit. These fractions are significantly higher than the 26% reported in the
optical bands for the same parent sample. We also present the fraction of
BALQSOs as functions of apparent magnitudes, absolute magnitudes, and redshift
in the 2MASS and SDSS bands. The 2MASS fractions are consistently higher than
the SDSS fractions in every comparison, and the BALQSO fractions steadily
increase with wavelength from the SDSS u to the 2MASS K_s bands. Furthermore,
the i - K_s color distributions of BALQSOs and non-BALQSOs indicate that
BALQSOs are redder than non-BALQSOs, with a K-S test probability of 2e-12.
These results are consistent with the spectral difference between BALQSOs and
non-BALQSOs including both the absorption troughs and dust extinction in
BALQSOs, which leads to significant selection biases against BALQSOs in the
optical bands. Using a simple simulation incorporating the luminosity function
of quasars and the amount of obscuration for BALQSOs, we simultaneously fit the
BALQSO fractions in the SDSS and 2MASS bands. We obtain a true BALQSO fraction
of 43\pm2% for luminous quasars (M_{K_s} \lesssim -30.1 mag).Comment: 20 pages, 6 figures, accepted by Ap
Seeking for the leading actor on the cosmic stage: Galaxies versus Supermassive Black Holes
We present a Special Issue on the interplay of galaxies and Supermassive
Black Holes (SMBHs) recently published in Advances in Astronomy. This is the
introductory paper containing the motivation for this Special Issue together
with a brief description of the articles which are part of the manuscript and
the link to the entire book (http://www.hindawi.com/journals/aa/si/610485/). We
hope this Special Issue will be useful for many astronomers who want to get an
update on the current status of the AGN-Galaxy coevolution topic.Comment: 4 pages, published in Advances in Astronomy as Introductory paper to
the Special Issue "Seeking for the leading actor on the cosmic stage:
Galaxies versus SMBHs". The entire book can be downloaded as PDF here:
http://www.hindawi.com/journals/aa/si/610485
Supermassive Black Holes and Their Host Galaxies - II. The correlation with near-infrared luminosity revisited
We present an investigation of the scaling relations between Supermassive
Black Hole (SMBH) masses (Mbh), and their host galaxies' K-band bulge (Lbul)
and total (Ltot) luminosities. The wide-field WIRCam imager at the
Canada-France-Hawaii-Telescope (CFHT) was used to obtain the deepest and
highest resolution near infrared images available for a sample of 35 galaxies
with securely measured Mbh, selected irrespective of Hubble type. For each
galaxy, we derive bulge and total magnitudes using a two-dimensional image
decomposition code that allows us to account, if necessary, for large- and
small-scale disks, cores, bars, nuclei, rings, envelopes and spiral arms. We
find that the present-day Mbh-Lbul and Mbh-Ltot relations have consistent
intrinsic scatter, suggesting that Mbh correlates equally well with bulge and
total luminosity of the host. Our analysis provides only mild evidence of a
decreased scatter if the fit is restricted to elliptical galaxies. The
log-slopes of the Mbh-Lbul and Mbh-Ltot relations are 0.75+/-0.10 and
0.92+/-0.14, respectively. However, while the slope of the Mbh-Lbul relation
depends on the detail of the image decomposition, the characterization of
Mbh-Ltot does not. Given the difficulties and ambiguities of decomposing galaxy
images into separate components, our results indicate that Ltot is more
suitable as a tracer of SMBH mass than Lbul, and that the Mbh-Ltot relation
should be used when studying the co-evolution of SMBHs and galaxies.Comment: 19 pages, 3 figures, 7 table
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