22 research outputs found
Growth of massive black holes at their late stage
We derive the black hole mass density as a function of redshift with the
bolometric luminosity function of AGN assuming that massive black holes grew
via accreting the circumnuclear gases, in which the derived black hole mass
density is required to match the measured local black hole mass density at z=0.
ADAFs are supposed to present in low luminosity AGNs/normal galaxies, which are
very hot and radiate mostly in the hard X-ray band. Most of the XRB is
contributed by bright AGNs, and a variety of AGN population synthesis models
were developed to model the observed XRB in the last two decades. Based on our
derived black hole mass density, we calculate the contribution to the XRB from
the ADAFs in faint AGNs/normal galaxies with a given Eddington ratio
distribution, which is mostly in hard X-ray energy band with an energy peak at
~200 keV. The growth of massive black holes during ADAF phase can therefore be
constrained with the observed XRB. Combining an AGN population synthesis model
with our results, we find that the fitting on the observed XRB, especially at
hard X-ray energy band with \ga 100 keV, is improved provided the contribution
of the ADAFs in low luminosity AGNs/normal galaxies is properly included. It is
found that less than ~15 per cent of local massive black hole mass density was
accreted during ADAF phases. We suggest that more accurate measurements of the
XRB in the energy band with \ga 100 keV in the future may help constrain the
growth of massive black holes at their late stage. We also calculate their
contribution to the extragalactic gamma-ray background, and find that less than
~1% of the observed EGRB is contributed by the ADAFs in these faint sources.Comment: 9 pages, 11 figures, accepted for publication in ApJ
Black Holes in Galaxy Mergers: Evolution of Quasars
Based on numerical simulations of gas-rich galaxy mergers, we discuss a model
in which quasar activity is tied to the self-regulated growth of supermassive
black holes in galaxies. Nuclear inflow of gas attending a galaxy collision
triggers a starburst and feeds black hole growth, but for most of the duration
of the starburst, the black hole is heavily obscured by surrounding gas and
dust which limits the visibility of the quasar, especially at optical and UV
wavelengths. Eventually, feedback energy from accretion heats the gas and
expels it in a powerful wind, leaving a 'dead quasar'. Between buried and dead
phases there is a window during which the galaxy would be seen as a luminous
quasar. Because the black hole mass, radiative output, and distribution of
obscuring gas and dust all evolve strongly with time, the duration of this
phase of observable quasar activity depends on both the waveband and imposed
luminosity threshold. We determine the observed and intrinsic lifetimes as a
function of luminosity and frequency, and calculate observable lifetimes ~10
Myr for bright quasars in the optical B-band, in good agreement with empirical
estimates and much smaller than the black hole growth timescales ~100 Myr,
naturally producing a substantial population of 'buried' quasars. However,
observed and intrinsic energy outputs converge in the IR and hard X-ray bands
as attenuation becomes weaker and chances of observation greatly increase. We
obtain the distribution of column densities along sightlines in which the
quasar is seen above a given luminosity, and find that our result agrees
remarkably well with observed estimates of the column density distribution from
the SDSS for appropriate luminosity thresholds. (Abridged)Comment: 12 pages, 7 figures. Accepted for publication in ApJ (September
2005). Replacement with minor revisions from referee repor
Constraints on QSO models from a relation between the QSO luminosity function and the local black hole mass function
QSOs are believed to be powered by accretion onto massive black holes (BHs).
In this paper, assuming that each central BH in nearby galaxies has experienced
the QSO phase and ignoring BH mergers, we establish a relation between the QSO
luminosity function (LF) and the local BH mass function (MF). The QSOLF is
jointly controlled by the luminosity evolution of individual QSOs and the
triggering history of the accretion onto seed BHs. By comparing the time
integral of the QSOLF with that inferred from local BHs, we separate the effect
of the luminosity evolution of individual QSOs from the effect of the
triggering history. Assuming that the nuclear luminosity evolution includes two
phases (first increasing at the Eddington luminosity with growth of BHs and
then declining), we find that observations are generally consistent with the
expected relation between the QSOLF and the local BHMF and obtain the
constraints on QSO models and BH growth. We point out that it is hard to
accurately estimate the value of the QSO lifetime from the QSOLF and/or the
local BHMF, if it is longer than a certain value (e.g., four times of the
Salpeter timescale in this study). We discuss the importance of accurate
measurements of the intrinsic scatter in the BH mass and velocity dispersion
relation of local galaxies and the scatter in the bolometric correction of
QSOs. We also discuss some possible applications of the work in this paper,
such as to the study of the demography of QSOs and the demography of normal
galaxies at intermediate redshift (abridged).Comment: 36 pages, including 9 figures; minor revisions to match the published
versio
Faint AGN and the Ionizing Background
We determine the evolution of the faint, high-redshift, optical luminosity
function (LF) of AGN implied by several observationally-motivated models of the
ionizing background. Our results depend crucially on whether we use the total
ionizing rate measured by the proximity effect technique or the lower
determination from the flux decrement distribution of Ly alpha forest lines.
Assuming a faint-end LF slope of 1.58 and the SDSS estimates of the bright-end
slope and normalization, we find that the LF must break at M_B*=-24.2,-22.3,
-20.8 at z=3,4, 5 if we adopt the lower ionization rate and assume no stellar
contribution to the background. The break must occur at M_B*=-20.6,-18.7, -18.7
for the proximity effect background estimate. These values brighten by as much
as ~2 mag if high-z galaxies contribute to the background with an escape
fraction of ionizing photons consistent with recent estimates: f_e=0.16. By
comparing to faint AGN searches, we find that the typically-quoted proximity
effect estimates of the background imply an over-abundance of faint AGN (even
with f_e=1). Even adopting the lower bound on proximity effect measurements,
the stellar escape fraction must be high: f_e>0.2. Conversely, the lower flux-
decrement-derived background requires a limited stellar contribution: f_e<0.05.
Our derived LFs together with the locally-estimated black hole density suggest
that the efficiency of converting mass to light in optically-unobscured AGN is
somewhat lower than expected, <0.05. Comparison with similar estimates based on
X-ray counts suggests that more than half of all AGN are obscured in the
UV/optical. We also derive lower limits on typical AGN lifetimes and obtain
>10^7 yrs for favored cases.Comment: 19 pages, 16 figures. Accepted by Astrophysical Journa
Growth of structure seeded by primordial black holes
We discuss the possibilities for primordial black holes (PBHs) to grow via
the accretion of dark matter. In agreement with previous works, we find that
accretion during the radiation-dominated era does not lead to a significant
mass increase. However, during matter-domination, PBHs may grow by up to two
orders of magnitude in mass through the acquisition of large dark matter halos.
We discuss the possibility of PBHs being an important component in dark matter
halos of galaxies as well as their potential to explain the ultra-luminous
x-ray sources (ULXs) observed in nearby galactic disks. We point out that
although PBHs are ruled out as the dominant component of dark matter, there is
still a great deal of parameter space open to them playing a role in the
modern-day universe. For example, a primordial halo population of PBHs each at
making up 0.1% of the dark matter grow to
via the accumulation of dark matter halos to account for of the dark
matter mass by a redshift of . These intermediate mass black
holes may then ``light up'' when passing through molecular clouds, becoming
visible as ULXs at the present day, or they may form the seeds for supermassive
black holes at the centers of galaxies.Comment: 10 pages, 5 figures. Submitted to Ap
Billiard Representation for Multidimensional Cosmology with Intersecting p-branes near the Singularity
Multidimensional model describing the cosmological evolution of n Einstein
spaces in the theory with l scalar fields and forms is considered. When
electro-magnetic composite p-brane ansatz is adopted, and certain restrictions
on the parameters of the model are imposed, the dynamics of the model near the
singularity is reduced to a billiard on the (N-1)-dimensional Lobachevsky
space, N = n+l. The geometrical criterion for the finiteness of the billiard
volume and its compactness is used. This criterion reduces the problem to the
problem of illumination of (N-2)-dimensional sphere by point-like sources. Some
examples with billiards of finite volume and hence oscillating behaviour near
the singularity are considered. Among them examples with square and triangle
2-dimensional billiards (e.g. that of the Bianchi-IX model) and a 4-dimensional
billiard in ``truncated'' D = 11 supergravity model (without the Chern-Simons
term) are considered. It is shown that the inclusion of the Chern-Simons term
destroys the confining of a billiard.Comment: 27 pages Latex, 3 figs., submit. to Class. Quantum Gra
Dark Matter Accretion into Supermassive Black Holes
The relativistic accretion rate of dark matter by a black hole is revisited.
Under the assumption that the phase space density indicator,
, remains constant during the inflow, the
derived accretion rate can be higher up to five orders of magnitude than the
classical accretion formula, valid for non-relativistic and non-interacting
particles, when typical dark halo conditions are considered. For these typical
conditions, the critical point of the flow is located at distances of about
30-150 times the horizon radius. Application of our results to black hole seeds
hosted by halos issued from cosmological simulations indicate that dark matter
contributes to no more than ~10% of the total accreted mass, confirming that
the bolometric quasar luminosity is related to the baryonic accretion history
of the black hole.Comment: 7 pages, 6 figures. Accepted for publication in Phys.Rev.
Supermassive black holes from primordial black hole seeds
The observational evidence for a population of quasars powered by
supermassive black holes of mass \geq 10^9 M_sun at redshifts z\geq 6 poses a
great challenge for any model describing the formation of galaxies. Assuming
uninterrupted accretion at the Eddington limit, seed black holes of at least
1000 M_sun are needed at z \approx 15. Here I study whether these seeds could
be primordial black holes (PBHs) which have been produced in the very early
universe by the collapse of primordial density fluctuations. In particular, I
study the expected number densities of PBHs in the relevant mass range for
several classes of spectra of primordial density fluctuations and confront the
results with observational data. While it seems to be possible to produce the
required PBHs with spectra showing large enhancements of fluctuations on a
certain scale, our hypothesis can be clearly disproved for a scale free
spectrum of primordial fluctuations described by a power-law slope consistent
with recent observations.Comment: 9 pages, 3 figures, minor changes, references added, accepted for
publication in the September 2004 issue of Phys. Rev.