106 research outputs found
Keplerian Motion of Broad-Line Region Gas as Evidence for Supermassive Black Holes in Active Galactic Nuclei
Emission-line variability data on NGC 5548 argue strongly for the existence
of a mass of order 7 x 10^7 solar masses within the inner few light days of the
nucleus in the Seyfert 1 galaxy NGC 5548. The time-delayed response of the
emission lines to continuum variations is used to infer the size of the
line-emitting region, and these determinations are combined with measurements
of the Doppler widths of the variable line components to estimate a virial
mass. The data for several different emission lines spanning an order of
magnitude in distance from the central source show the expected V proportional
to r^{-1/2} correlation and are consistent with a single value for the mass.Comment: 9 pages, 2 Figures. accepted by ApJ Letter
The Black Hole to Bulge Mass Relation in Active Galactic Nuclei
The masses of the central black holes in Active Galactic Nuclei (AGNs) can be
estimated using the broad emission-lines as a probe of the virial mass. Using
reverberation mapping to determine the size of the Broad Line Region (BLR) and
the width of the variable component of the line profile H line it is
possible to find quite accurate virial mass estimates for AGNs with adequate
data. Compiling a sample of AGNs with reliable central masses and bulge
magnitudes we find an average black-hole-to-bulge mass ratio of 0.0003, a
factor of 20 less than the value found for normal galaxies and for bright
quasars. This lower ratio is more consistent with the back hole mass density
predicted from quasar light, and is similar to the central black hole/bulge
mass ratio in our Galaxy. We argue that the black hole/bulge mass ratio
actually has a significantly larger range than indicated by mssive black holes
detected in normal galaxies (using stellar dynamics) and in bright quasars,
which may be biased towards large black holes. We derive a scenario of black
hole growth that explains the observed distribution.Comment: 12 pages LaTeX, including 2 revised figures, revised table. Revised
version to be published in the Astrophysical Journal (Letters) Ap.J.Lett. 51
Evidence for Supermassive Black Holes in Active Galactic Nuclei from Emission-Line Reverberation
Emission-line variability data for Seyfert 1 galaxies provide strong evidence
for the existence of supermassive black holes in the nuclei of these galaxies,
and that the line-emitting gas is moving in the gravitational potential of that
black hole. The time-delayed response of the emission lines to continuum
variations is used to infer the size of the line-emitting region, which is then
combined with measurements of the Doppler widths of the variable line
components to estimate a virial mass. In the case of the best-studied galaxy,
NGC 5548, various emission lines spanning an order of magnitude in distance
from the central source show the expected velocity proportional to inverse
square root of the distance correlation between distance and line width, and
are thus consistent with a single value for the mass. Two other Seyfert
galaxies, NGC 7469 and 3C 390.3, show a similar relationship. We compute the
ratio of luminosity to mass for these three objects and the narrow-line Seyfert
1 galaxy NGC 4051 and find that that the gravitational force on the
line-emitting gas is much stronger than radiation pressure. These results
strongly support the paradigm of gravitationally bound broad emission-line
region clouds.Comment: 10 pages, 2 figures, Accepted for publication in Astrophysical
Journal Letter
The Cosmic Density of Massive Black Holes from Galaxy Velocity Dispersions
Supermassive black holes are thought to be relics of quasars, and their
numbers and masses are therefore related to the quasar luminosity function and
its evolution with redshift. We have used the relationship between black hole
mass and bulge velocity dispersion (the M_bullet - sigma relation) to make an
improved estimate of the mass density and mass spectrum of supermassive black
holes. Uncertainties in the M_bullet - sigma relation have little effect on the
mass density. We find a mass density of (4.8 +/- 1.6) h^2 x 10^5 M_sun Mpc^-3.
Some of the variance in published density estimates comes from the use of
different values of the Hubble constant.Comment: To appear in the December 2002 issue of The Astronomical Journa
A Theoretical Model for the Relation for Supermassive Black Holes in Galaxies
We construct a model for the formation of black holes within galactic bulges.
The initial state is a slowly rotating isothermal sphere, characterized by
effective transport speed \aeff and rotation rate . The black hole
mass is determined when the centrifugal radius of the collapse flow exceeds the
capture radius of the central black hole. This model reproduces the observed
correlation between black hole masses and galactic velocity dispersions, \mbh
\approx 10^8 M_\odot (\sigma/200 \kms)^4, where \sigma = \sqrt{2} \aeff.
This model also predicts the ratio \mrat of black hole mass to host mass:
\mrat 0.004 (\sigma/200 \kms).Comment: 9 pages, 2 figures, submitted to Astrophysical Journal Letter
Star Captures by Quasar Accretion Disks: A Possible Explanation of the M-sigma Relation
A new theory of quasars is presented in which the matter of thin accretion
disks around black holes is supplied by stars that plunge through the disk.
Stars in the central part of the host galaxy are randomly perturbed to highly
radial orbits, and as they repeatedly cross the disk they lose orbital energy
by drag, eventually merging into the disk. Requiring the rate of stellar mass
capture to equal the mass accretion rate into the black hole, a relation
between the black hole mass and the stellar velocity dispersion is predicted of
the form M_{BH} \propto sigma_*^{30/7}. The normalization depends on various
uncertain parameters such as the disk viscosity, but is consistent with
observation for reasonable assumptions. We show that a seed central black hole
in a newly formed stellar system can grow at the Eddington rate up to this
predicted mass via stellar captures by the accretion disk. Once this mass is
reached, star captures are insufficient to maintain an Eddington accretion
rate, and the quasar may naturally turn off as the accretion switches to a
low-efficiency advection mode. The model provides a mechanism to deliver mass
to the accretion disk at small radius, probably solving the problem of
gravitational instability to star formation in the disk at large radius. We
note that the matter from stars that is incorporated to the disk has an average
specific angular momentum that is very small or opposite to that of the disk,
and discuss how a rotating disk may be maintained as it captures this matter if
a small fraction of the accreted mass comes from stellar winds that form a disk
extending to larger radius. We propose several observational tests and
consequences of this theory.Comment: submitted to Ap
Core depletion from coalescing supermassive black holes
New measurements of the stellar-mass deficits at the centers of luminous
elliptical galaxies are presented. These were derived considering the following
observational facts. Firstly, ``core'' galaxies, which are thought to have had
their inner region depleted from the coalesence of supermassive black holes,
show an abrupt downward deviation of their inner light-profile relative to
their outer Sersic profile. Second, ``power-law'' galaxies, having undisturbed
profiles and no partially depleted core, have inner light-profiles that display
no departure from the inward extrapolation of their outer Sersic profile. The
central stellar deficits have therefore been derived from the difference in
flux between the HST-observed galaxy light-profiles and the inward
extrapolation of each galaxy's outer Sersic profile. This approach gives flux
deficits ~0.1% of the total galaxy light, and mass deficits that are ~2 times
each galaxy's central supermassive black hole mass. These results are in
agreement with the theoretical expectations of mass ejection from binary black
hole mergers and also with popular LCDM models of hierarchical galaxy
formation. It is also explained why this result is some 10 times smaller than
current observational estimates of the central mass deficit, and therefore
implies a merger history for giant elliptical galaxies that is one order of
magnitude less violent than previously suggested.Comment: 7 pages, ApJ Letter
Clustering Analyses of 300,000 Photometrically Classified Quasars--II. The Excess on Very Small Scales
We study quasar clustering on small scales, modeling clustering amplitudes
using halo-driven dark matter descriptions. From 91 pairs on scales <35 kpc/h,
we detect only a slight excess in quasar clustering over our best-fit
large-scale model. Integrated across all redshifts, the implied quasar bias is
b_Q = 4.21+/-0.98 (b_Q = 3.93+/-0.71) at ~18 kpc/h (~28 kpc/h). Our best-fit
(real-space) power index is ~-2 (i.e., ), implying
steeper halo profiles than currently found in simulations. Alternatively,
quasar binaries with separation <35 kpc/h may trace merging galaxies, with
typical dynamical merger times t_d~(610+/-260)m^{-1/2} Myr/h, for quasars of
host halo mass m x 10^{12} Msolar/h. We find UVX quasars at ~28 kpc/h cluster
>5 times higher at z > 2, than at z < 2, at the level. However, as
the space density of quasars declines as z increases, an excess of quasar
binaries (over expectation) at z > 2 could be consistent with reduced merger
rates at z > 2 for the galaxies forming UVX quasars. Comparing our clustering
at ~28 kpc/h to a \xi(r)=(r/4.8\Mpch)^{-1.53} power-law, we find an upper
limit on any excess of a factor of 4.3+/-1.3, which, noting some caveats,
differs from large excesses recently measured for binary quasars, at
. We speculate that binary quasar surveys that are biased to z > 2
may find inflated clustering excesses when compared to models fit at z < 2. We
provide details of 111 photometrically classified quasar pairs with separations
<0.1'. Spectroscopy of these pairs could significantly constrain quasar
dynamics in merging galaxies.Comment: 12pages, 3 figures, 2 tables; uses amulateapj; accepted to Ap
Nuclear Cusps and Cores in Early-type Galaxies As Relics of Binary Black Hole Mergers
We present an analysis of the central cusp slopes and core parameters of
early-type galaxies using a large database of surface brightness profiles
obtained from Hubble Space Telescope observations. We examine the relation
between the central cusp slopes, core parameters, and black hole masses in
early-type galaxies, in light of two models that attempt to explain the
formation of cores and density cusps via the dynamical influence of black
holes. Contrary to the expectations from adiabatic-growth models, we find that
the cusp slopes do not steepen with increasing black hole mass fraction.
Moreover, a comparison of kinematic black hole mass measurements with the
masses predicted by the adiabatic models shows that they overpredict the masses
by a factor of approximately 3. Simulations involving binary black hole mergers
predict that both the size of the core and the central mass deficit correlate
with the final black hole mass. These relations are qualitatively supported by
the present data.Comment: To appear in ApJ. 8 page
Formation of Supermassive Black Holes in Galactic Bulges: A Rotating Collapse Model Consistent with the \mbh-\sigma Relation
Motivated by the observed correlation between black hole masses \mbh and
the velocity dispersion of host galaxies, we develop a theoretical
model of black hole formation in galactic bulges (this paper generalizes an
earlier ApJ Letter). The model assumes an initial state specified by a a
uniform rotation rate and a density distribution of the form \rho =
\aeff^2 / 2 \pi G r^2 (so that \aeff is an effective transport speed). The
black hole mass is determined when the centrifugal radius of the collapse flow
exceeds the capture radius of the central black hole (for Schwarzschild
geometry). This model reproduces the observed correlation between the estimated
black hole masses and the velocity dispersions of galactic bulges, i.e., \mbh
\approx 10^8 M_\odot (\sigma/200 {\rm km s^{-1}})^4, where \sigma = \sqrt{2}
\aeff. To obtain this normalization, the rotation rate rad/s. The model also defines a bulge mass scale . If we
identify the scale with the bulge mass, the model determines the ratio
\mrat of black hole mass to the host mass: \mrat 0.0024
, again in reasonable agreement with observed
values. In this scenario, supermassive black holes form quickly (in
yr) and are born rapidly rotating (with ). This paper also shows
how these results depend on the assumed initial conditions; the most important
quantity is the initial distribution of specific angular momentum in the
pre-collapse state.Comment: 31 pages, 4 figures, accepted to Ap
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