882 research outputs found
Supermassive Black Holes Then and Now
Recent surveys suggest that most or all normal galaxies host a massive black
hole with 1/100 to 1/1000 of the visible mass of the spheroid of the galaxy.
Various lines of argument suggest that these galaxies have merged at least once
in our past lightcone, and that the black holes have also merged. This leads to
a merger rate of massive black holes of about 1/\yrs.Comment: 7 pages, to appear in The Proceedings of the Second International
LISA Symposium on Graviational Waves, ed. W. Folkne
NGC 1300 Dynamics: II. The response models
We study the stellar response in a spectrum of potentials describing the
barred spiral galaxy NGC 1300. These potentials have been presented in a
previous paper and correspond to three different assumptions as regards the
geometry of the galaxy. For each potential we consider a wide range of
pattern speed values. Our goal is to discover the geometries and the
supporting specific morphological features of NGC 1300. For this
purpose we use the method of response models. In order to compare the images of
NGC 1300 with the density maps of our models, we define a new index which is a
generalization of the Hausdorff distance. This index helps us to find out
quantitatively which cases reproduce specific features of NGC 1300 in an
objective way. Furthermore, we construct alternative models following a
Schwarzschild type technique. By this method we vary the weights of the various
energy levels, and thus the orbital contribution of each energy, in order to
minimize the differences between the response density and that deduced from the
surface density of the galaxy, under certain assumptions. We find that the
models corresponding to \ksk and \ksk are
able to reproduce efficiently certain morphological features of NGC 1300, with
each one having its advantages and drawbacks.Comment: 13 pages, 10 figures, accepted for publication in MNRA
Measuring the Radiative Histories of QSOs with the Transverse Proximity Effect
Since the photons that stream from QSOs alter the ionization state of the gas
they traverse, any changes to a QSO's luminosity will produce
outward-propagating ionization gradients in the surrounding intergalactic gas.
This paper shows that at redshift z~3 the gradients will alter the gas's
Lyman-alpha absorption opacity enough to produce a detectable signature in the
spectra of faint background galaxies. By obtaining noisy (S:N~4) low-resolution
(~7A) spectra of a several dozen background galaxies in an R~20' field
surrounding an isotropically radiating 18th magnitude QSO at z=3, it should be
possible to detect any order-of-magnitude changes to the QSO's luminosity over
the previous 50--100 Myr and to measure the time t_Q since the onset of the
QSO's current luminous outburst with an accuracy of ~5 Myr for t_Q<~50 Myr.
Smaller fields-of-view are acceptable for shorter QSO lifetimes. The major
uncertainty, aside from cosmic variance, will be the shape and orientation of
the QSO's ionization cone. This can be determined from the data if the number
of background sources is increased by a factor of a few. The method will then
provide a direct test of unification models for AGN.Comment: Accepted for publication in the ApJ. 16 page
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
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
Hypervelocity binary stars: smoking gun of massive binary black holes
The hypervelocity stars recently found in the Galactic halo are expelled from
the Galactic center through interactions between binary stars and the central
massive black hole or between single stars and a hypothetical massive binary
black hole. In this paper, we demonstrate that binary stars can be ejected out
of the Galactic center with velocities up to 10^3 km/s, while preserving their
integrity, through interactions with a massive binary black hole. Binary stars
are unlikely to attain such high velocities via scattering by a single massive
black hole or through any other mechanisms. Based on the above theoretical
prediction, we propose a search for binary systems among the hypervelocity
stars. Discovery of hypervelocity binary stars, even one, is a definitive
evidence of the existence of a massive binary black hole in the Galactic
center.Comment: 5 pages, 3 figures, shortened version, ApJL in pres
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
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