707 research outputs found
Intermediate mass black holes in AGN disks: I. Production & Growth
Here we propose a mechanism for efficiently growing intermediate mass black
holes (IMBH) in disks around supermassive black holes. Stellar mass objects can
efficiently agglomerate when facilitated by the gas disk. Stars, compact
objects and binaries can migrate, accrete and merge within disks around
supermassive black holes. While dynamical heating by cusp stars excites the
velocity dispersion of nuclear cluster objects (NCOs) in the disk, gas in the
disk damps NCO orbits. If gas damping dominates, NCOs remain in the disk with
circularized orbits and large collision cross-sections. IMBH seeds can grow
extremely rapidly by collisions with disk NCOs at low relative velocities,
allowing for super-Eddington growth rates. Once an IMBH seed has cleared out
its feeding zone of disk NCOs, growth of IMBH seeds can become dominated by gas
accretion from the AGN disk. However, the IMBH can migrate in the disk and
expand its feeding zone, permitting a super-Eddington accretion rate to
continue. Growth of IMBH seeds via NCO collisions is enhanced by a pile-up of
migrators.
We highlight the remarkable parallel between the growth of IMBH in AGN disks
with models of giant planet growth in protoplanetary disks. If an IMBH becomes
massive enough it can open a gap in the AGN disk. IMBH migration in AGN disks
may stall, allowing them to survive the end of the AGN phase and remain in
galactic nuclei. Our proposed mechanisms should be more efficient at growing
IMBH in AGN disks than the standard model of IMBH growth in stellar clusters.
Dynamical heating of disk NCOs by cusp stars is transferred to the gas in a AGN
disk helping to maintain the outer disk against gravitational instability.
Model predictions, observational constraints and implications are discussed in
a companion paper (Paper II).Comment: 11 pages, 4 figures, MNRAS (accepted
On rapid migration and accretion within disks around supermassive black holes
Galactic nuclei should contain a cluster of stars and compact objects in the
vicinity of the central supermassive black hole due to stellar evolution, minor
mergers and gravitational dynamical friction. By analogy with protoplanetary
migration, nuclear cluster objects (NCOs) can migrate in the accretion disks
that power active galactic nuclei by exchanging angular momentum with disk gas.
Here we show that an individual NCO undergoing runaway outward migration
comparable to Type III protoplanetary migration can generate an accretion rate
corresponding to Seyfert AGN or quasar luminosities. Multiple migrating NCOs in
an AGN disk can dominate traditional viscous disk accretion and at large disk
radii, ensemble NCO migration and accretion could provide sufficient heating to
prevent the gravitational instability from consuming disk gas in star
formation. The magnitude and energy of the X-ray soft excess observed at
~0.1-1keV in Seyfert AGN could be explained by a small population of
~10^{2}-10^{3} accreting stellar mass black holes or a few ULXs. NCO migration
and accretion in AGN disks are therefore extremely important mechanisms to add
to realistic models of AGN disks.Comment: 6 pages, 2 figures, MNRAS Letters (accepted
Black Hole Mass, Host galaxy classification and AGN activity
We investigate the role of host galaxy classification and black hole mass in
a heterogeneous sample of 276 mostly nearby (z<0.1) X-ray and IR selected AGN.
Around 90% of Seyfert 1 AGN in bulge-dominated host galaxies (without disk
contamination) span a very narrow range in the observed 12um to 2-10keV
luminosity ratio (1<R_{IR/X}<7). This narrow dispersion incorporates all
possible variations among AGN central engines, including accretion mechanism
and efficiency, disk opening angle, orientation to sightline, covering fraction
of absorbing material, patchiness of X-ray corona and measured variability. As
a result, all models of X-ray and IR production in AGN are very strongly
constrained. Among Seyfert 1 AGN, median X-ray and IR luminosities increase
with black hole mass at >99% confidence. Using ring morphology of the host
galaxy as a proxy for lack of tidal interaction, we find that AGN luminosity in
host galaxies within 70Mpc is independent of host galaxy interaction for
Gyrs, suggesting that the timescale of AGN activity due to secular evolution is
much shorter than that due to tidal interactions. We find that LINER hosts have
lower 12um luminosity than the median 12um luminosity of normal disk- and
bulge-dominated galaxies which may represent observational evidence for past
epochs of feedback that supressed star formation in LINER host galaxies. We
propose that nuclear ULXs may account for the X-ray emission from LINER 2s
without flat-spectrum, compact radio cores. We confirmed the robustness of our
results in X-rays by comparing them with the 14-195keV 22-month BAT survey of
AGN, which is all-sky and unbiased by photoelectric absorption.Comment: MNRAS accepted. 14 pages, 11 figures, complete Table 1 in online
journa
- …