637 research outputs found
Assessing inflow rates in atomic cooling halos: implications for direct collapse black holes
Supermassive black holes are not only common in the present-day galaxies, but
billion solar masses black holes also powered quasars. One efficient
way to form such black holes is the collapse of a massive primordial gas cloud
into a so-called direct collapse black hole. The main requirement for this
scenario is the presence of large accretion rates of to form a supermassive star. It is not yet clear how and
under what conditions such accretion rates can be obtained. The prime aim of
this work is to determine the mass accretion rates under non-isothermal
collapse conditions. We perform high resolution cosmological simulations for
three primordial halos of a few times illuminated by an
external UV flux, . We find that a rotationally supported
structure of about parsec size is assembled, with an aspect ratio between depending upon the thermodynamical properties. Rotational support,
however, does not halt collapse, and mass inflow rates of can be obtained in the presence of even a moderate UV
background flux of strength . To assess whether such large
accretion rates can be maintained over longer time scales, we employed sink
particles, confirming the persistence of accretion rates of . We propose that complete isothermal collapse and molecular
hydrogen suppression may not always be necessary to form supermassive stars,
precursors of black hole seeds. Sufficiently high inflow rates can be obtained
for UV flux , at least for some cases. This value brings
the estimate of the abundance of direct collapse black hole seeds closer to
that high redshift quasars.Comment: Accepted for publication in MNRAS, comments are still welcom
The Merging History of Massive Black Holes
We investigate a hierarchical structure formation scenario describing the
evolution of a Super Massive Black Holes (SMBHs) population. The seeds of the
local SMBHs are assumed to be 'pregalactic' black holes, remnants of the first
POPIII stars. As these pregalactic holes become incorporated through a series
of mergers into larger and larger halos, they sink to the center owing to
dynamical friction, accrete a fraction of the gas in the merger remnant to
become supermassive, form a binary system, and eventually coalesce. A simple
model in which the damage done to a stellar cusps by decaying BH pairs is
cumulative is able to reproduce the observed scaling relation between galaxy
luminosity and core size. An accretion model connecting quasar activity with
major mergers and the observed BH mass-velocity dispersion correlation
reproduces remarkably well the observed luminosity function of
optically-selected quasars in the redshift range 1<z<5. We finally asses the
potential observability of the gravitational wave background generated by the
cosmic evolution of SMBH binaries by the planned space-born interferometer
LISA.Comment: 4 pages, 2 figures, Contribute to "Multiwavelength Cosmology",
Mykonos, Greece, June 17-20, 200
Unresolved X-ray background: clues on galactic nuclear activity at z>6
We study, by means of dedicated simulations of massive black hole build-up,
the possibility to constraint the existence and nature of the AGN population at
z>6 with available and planned X-ray and near infrared space telescopes. We
find that X-ray deep-field observations can set important constraints to the
faint-end of the AGN luminosity function at very high redshift. Planned X-ray
telescopes should be able to detect AGN hosting black holes with masses down to
>10^5 Msun (i.e., X-ray luminosities in excess of 10^42 erg s^-1), and can
constrain the evolution of the population of massive black hole at early times
(6<z<10). We find that this population of AGN should contribute substantially
(~25%) to the unresolved fraction of the cosmic X-ray background in the 0.5-10
keV range, and that a significant fraction (~3-4%) of the total background
intensity would remain unaccounted even after future X-ray observations. As
byproduct, we compute the expected UV background from AGN at z>6 and we discuss
the possible role of AGN in the reionization of the Universe at these early
epochs, showing that AGN alone can provide enough ionizing photons only in the
(improbable) case of an almost completely homogeneous inter-galactic medium.
Finally, we show that super-Eddington accretion, suggested by the observed QSOs
at z>6, must be a very rare event, confined to black holes living in the
highest density peaks.Comment: 9 pages, 7 figures, MNRAS in pres
The Formation and Evolution of Massive Black Holes
The past 10 years have witnessed a change of perspective in the way
astrophysicists think about massive black holes (MBHs), which are now
considered to have a major role in the evolution of galaxies. This appreciation
was driven by the realization that black holes of millions solar masses and
above reside in the center of most galaxies, including the Milky Way. MBHs also
powered active galactic nuclei known to exist just a few hundred million years
after the Big Bang. Here, I summarize the current ideas on the evolution of
MBHs through cosmic history, from their formation about 13 billion years ago to
their growth within their host galaxies.Comment: Review for Science Special Issue on black hole
Cosmography with strong lensing of LISA gravitational wave sources
LISA might detect gravitational waves from mergers of massive black hole
binaries strongly lensed by intervening galaxies (Sereno et al. 2010). The
detection of multiple gravitational lensing events would provide a new tool for
cosmography. Constraints on cosmological parameters could be placed by
exploiting either lensing statistics of strongly lensed sources or time delay
measurements of lensed gravitational wave signals. These lensing methods do not
need the measurement of the redshifts of the sources and the identification of
their electromagnetic counterparts. They would extend cosmological probes to
redshift z <= 10 and are then complementary to other lower or higher redshift
tests, such as type Ia supernovae or cosmic microwave background. The accuracy
of lensing tests strongly depends on the formation history of the merging
binaries, and the related number of total detectable multiple images. Lensing
amplification might also help to find the host galaxies. Any measurement of the
source redshifts would allow to exploit the distance-redshift test in
combination with lensing methods. Time-delay analyses might measure the Hubble
parameter H_0 with accuracy of >= 10 km s^{-1}Mpc^{-1}. With prior knowledge of
H_0, lensing statistics and time delays might constrain the dark matter density
(delta Omega_M >= 0.08, due to parameter degeneracy). Inclusion of our methods
with other available orthogonal techniques might significantly reduce the
uncertainty contours for Omega_M and the dark energy equation of state.Comment: 10 pages, 10 figures, in press on MNRA
On the orientation and magnitude of the black hole spin in galactic nuclei
Massive black holes in galactic nuclei vary their mass M and spin vector J
due to accretion. In this study we relax, for the first time, the assumption
that accretion can be either chaotic, i.e. when the accretion episodes are
randomly and isotropically oriented, or coherent, i.e. when they occur all in a
preferred plane. Instead, we consider different degrees of anisotropy in the
fueling, never confining to accretion events on a fixed direction. We follow
the black hole growth evolving contemporarily mass, spin modulus a and spin
direction. We discover the occurrence of two regimes. An early phase (M <~ 10
million solar masses) in which rapid alignment of the black hole spin direction
to the disk angular momentum in each single episode leads to erratic changes in
the black hole spin orientation and at the same time to large spins (a ~ 0.8).
A second phase starts when the black hole mass increases above >~ 10 million
solar masses and the accretion disks carry less mass and angular momentum
relatively to the hole. In the absence of a preferential direction the black
holes tend to spin-down in this phase. However, when a modest degree of
anisotropy in the fueling process (still far from being coherent) is present,
the black hole spin can increase up to a ~ 1 for very massive black holes (M >~
100 million solar masses), and its direction is stable over the many accretion
cycles. We discuss the implications that our results have in the realm of the
observations of black hole spin and jet orientations.Comment: 14 pages, 7 figures, accepted for publication in Ap
A path to radio-loudness through gas-poor galaxy mergers and the role of retrograde accretion
In this proceeding we explore a pathway to radio-loudness under the
hypothesis that retrograde accretion onto giant spinning black holes leads to
the launch of powerful jets, as seen in radio loud QSOs and recently in
LAT/Fermi and BAT/Swift Blazars. Counter-rotation of the accretion disc
relative to the BH spin is here associated to gas-poor galaxy mergers
progenitors of giant (missing-light) ellipticals. The occurrence of retrograde
accretion enters as unifying element that may account for the
radio-loudness/galaxy morphology dichotomy observed in AGN.Comment: To appear in the proceedings of the conference "Accretion and
Ejection in AGN: A global view, June 22-26 2009 - Como, Italy
New observational Constraints on the Growth of the First Supermassive Black Holes
We constrain the total accreted mass density in supermassive black holes at
z>6, inferred via the upper limit derived from the integrated X-ray emission
from a sample of photometrically selected galaxy candidates. Studying galaxies
obtained from the deepest Hubble Space Telescope images combined with the
Chandra 4 Msec observations of the Chandra Deep Field South, we achieve the
most restrictive constraints on total black hole growth in the early Universe.
We estimate an accreted mass density <1000Mo Mpc^-3 at z~6, significantly lower
than the previous predictions from some existing models of early black hole
growth and earlier prior observations. These results place interesting
constraints on early black growth and mass assembly by accretion and imply one
or more of the following: (1) only a fraction of the luminous galaxies at this
epoch contain active black holes; (2) most black hole growth at early epochs
happens in dusty and/or less massive - as yet undetected - host galaxies; (3)
there is a significant fraction of low-z interlopers in the galaxy sample; (4)
early black hole growth is radiatively inefficient, heavily obscured and/or is
due to black hole mergers as opposed to accretion or (5) the bulk of the black
hole growth occurs at late times. All of these possibilities have important
implications for our understanding of high redshift seed formation models.Comment: ApJ Accepted, 10 pages, 7 figures, 1 table, in emulateapj forma
Limits on the high redshift growth of massive black holes
We place firm upper limits on the global accretion history of massive black
holes at z>5 from the recently measured unresolved fraction of the cosmic X-ray
background. The maximum allowed unresolved intensity observed at 1.5 keV
implies a maximum accreted-mass density onto massive black holes of rho_acc <
1.4E4 M_sun Mpc^{-3} for z>5. Considering the contribution of lower-z AGNs, the
value reduces to rho_acc < 0.66E4 M_sun Mpc^{-3}. The tension between the need
for the efficient and rapid accretion required by the observation of massive
black holes already in place at z>7 and the strict upper limit on the accreted
mass derived from the X-ray background may indicate that black holes are rare
in high redshift galaxies, or that accretion is efficient only for black holes
hosted by rare galaxies.Comment: 5 pages, 1 figure, published in A&A Letter
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