8 research outputs found
AGN disks and black holes on the weighting scales
We exploit our formula for the gravitational potential of finite size,
power-law disks to derive a general expression linking the mass of the black
hole in active galactic nuclei (AGN), the mass of the surrounding disk, its
surface density profile (through the power index s), and the differential
rotation law. We find that the global rotation curve v(R) of the disk in
centrifugal balance does not obey a power law of the cylindrical radius R
(except in the confusing case s = -2 that mimics a Keplerian motion), and
discuss the local velocity index. This formula can help to understand how, from
position-velocity diagrams, mass is shared between the disk and the black hole.
To this purpose, we have checked the idea by generating a sample of synthetic
data with different levels of Gaussian noise, added in radius. It turns out
that, when observations are spread over a large radial domain and exhibit low
dispersion (standard deviation less than 10% typically), the disk properties
(mass and s-parameter) and black hole mass can be deduced from a non linear fit
of kinematic data plotted on a (R, Rv 2)-diagram. For a deviation higher than
10%, masses are estimated fairly well from a linear regression (corresponding
to the zeroth-order treatment of the formula), but the power index s is no
longer accessible. We have applied the model to 7 AGN disks whose rotation has
already been probed through water maser emission. For NGC3393 and UGC3789, the
masses seem well constrained through the linear approach. For IC1481, the
power-law exponent s can even be deduced. Because the model is scale-free, it
applies to any kind of star/disk system. Extension to disks around young stars
showing deviation from Keplerian motion is thus straightforward.Comment: accepted for publication in A&
Near-Infrared Reverberation by Dusty Clumpy Tori in Active Galactic Nuclei
According to recent models, the accretion disk and black hole in active
galactic nuclei are surrounded by a clumpy torus. We investigate the NIR flux
variation of the torus in response to a UV flash for various geometries.
Anisotropic illumination by the disk and the torus self-occultation contrast
our study with earlier works. Both the waning effect of each clump and the
torus self-occultation selectively reduce the emission from the region with a
short delay. Therefore, the NIR delay depends on the viewing angle (where a
more inclined angle leads to a longer delay) and the time response shows an
asymmetric profile with a negative skewness, opposing to the results for
optically thin tori. The range of the computed delay coincides with the
observed one, suggesting that the viewing angle is primarily responsible for
the scatter of the observed delay. We also propose that the red NIR-to-optical
color of type-1.8/1.9 objects is caused by not only the dust extinction but
also the intrinsically red color. Compared with the modest torus thickness,
both a thick and a thin tori display the weaker NIR emission. A selection bias
is thus expected such that NIR-selected AGNs tend to possess moderately thick
tori. A thicker torus shows a narrower and more heavily skewed time profile,
while a thin torus produces a rapid response. A super-Eddington accretion rate
leads to a much weaker NIR emission due to the disk self-occultation and the
disk truncation by the self-gravity. A long delay is expected from an optically
thin and/or a largely misaligned torus. A very weak NIR emission, such as in
hot-dust-poor active nuclei, can arise from a geometrically thin torus, a
super-Eddington accretion rate or a slightly misaligned torus.Comment: 15 pages, 15 figures, Accepted for publication in Ap
A Scaling Relation Between Megamaser Disk Radius and Black Hole Mass in Active Galactic Nuclei
Several thin, Keplerian, sub-parsec megamaser disks have been discovered in
the nuclei of active galaxies and used to precisely determine the mass of their
host black holes. We show that there is an empirical linear correlation between
the disk radius and the black hole mass. We demonstrate that such disks are
naturally formed by the partial capture of molecular clouds passing through the
galactic nucleus and temporarily engulfing the central supermassive black hole.
Imperfect cancellation of the angular momenta of the cloud material colliding
after passing on opposite sides of the hole leads to the formation of a compact
disk. The radial extent of the disk is determined by the efficiency of this
process and the Bondi-Hoyle capture radius of the black hole, and naturally
produces the empirical linear correlation of the radial extent of the maser
distribution with black hole mass. The disk has sufficient column density to
allow X-ray irradiation from the central source to generate physical and
chemical conditions conducive to the formation of 22 GHz H2O masers. For
initial cloud column densities less than ~10^{23.5} cm^-2 the disk is non-self
gravitating, consistent with the ordered kinematics of the edge-on megamaser
disks; for higher cloud columns the disk would fragment and produce a compact
stellar disk similar to that observed around Sgr A* at the galactic centre.Comment: Minor revisions. Fig 1. corrected. ApJ Letters, in pres
The Megamaser Cosmology Project. III. Accurate Masses of Seven Supermassive Black Holes in Active Galaxies with Circumnuclear Megamaser Disks
Observations of HO masers from circumnuclear disks in active galaxies for
the Megamaser Cosmology Project allow accurate measurement of the mass of
supermassive black holes (BH) in these galaxies. We present the Very Long
Baseline Interferometry (VLBI) images and kinematics of water maser emission in
six active galaxies: NGC~1194, NGC~2273, NGC~2960 (Mrk~1419), NGC~4388,
NGC~6264 and NGC~6323. We use the Keplerian rotation curves of these six
megamaser galaxies, plus a seventh previously published, to determine accurate
enclosed masses within the central pc of these galaxies, smaller than
the radius of the sphere of influence of the central mass in all cases. We also
set lower limits to the central mass densities of between 0.12 and 60 ~pc. For six of the seven disks, the high central
densities rule out clusters of stars or stellar remnants as the central
objects, and this result further supports our assumption that the enclosed mass
can be attributed predominantly to a supermassive black hole. The seven BHs
have masses ranging between 0.76 and 6.510. The BH mass
errors are \%, dominated by the uncertainty of the Hubble constant.
We compare the megamaser BH mass determination with other BH mass measurement
techniques. The BH mass based on virial estimation in four galaxies is
consistent with the megamaser BH mass given the latest empirical value of
, but the virial mass uncertainty is much greater. MCP
observations continue and we expect to obtain more maser BH masses in the
future.Comment: 18 pages, 4 figures. This paper has been submitted to ApJ. An updated
version of this paper will be posted when it gets accepte