194 research outputs found
Radiation Pressure Supported Starburst Disks and AGN Fueling
We consider the structure of marginally Toomre-stable starburst disks under
the assumption that radiation pressure on dust grains provides the dominant
vertical support against gravity. This is particularly appropriate when the
disk is optically thick to its own IR radiation, as in the central regions of
ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for
star formation changes in the optically-thick limit, with the star formation
rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface
density and kappa is the mean opacity. We show that optically thick starburst
disks have a characteristic flux and dust effective temperature of F ~ 10^{13}
L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with
observations and find good agreement. We extend our model from many-hundred
parsec scales to sub-parsec scales and address the problem of fueling AGN. We
assume that angular momentum transport proceeds via global torques rather than
a local viscosity. We account for the radial depletion of gas due to star
formation and find a strong bifurcation between two classes of disk models: (1)
solutions with a starburst on large scales that consumes all of the gas with
little fueling of a central AGN and (2) models with an outer large-scale
starburst accompanied by a more compact starburst on 1-10 pc scales and a
bright central AGN. The luminosity of the latter models is in many cases
dominated by the AGN. We show that the vertical thickness of the starburst disk
on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration
in some Type 2 AGN. We also argue that the disk of young stars in the Galactic
Center may be the remnant of such a compact nuclear starburst.Comment: 26 pages, 9 figures, emulateapj, accepted to ApJ, minor changes,
discussion tightened, references adde
An axisymmetric hydrodynamical model for the torus wind in AGN. II: X-ray excited funnel flow
We have calculated a series of models of outflows from the obscuring torus in
active galactic nuclei (AGN). Our modeling assumes that the inner face of a
rotationally supported torus is illuminated and heated by the intense X-rays
from the inner accretion disk and black hole. As a result of such heating a
strong biconical outflow is observed in our simulations. We calculate
3-dimensional hydrodynamical models, assuming axial symmetry, and including the
effects of X-ray heating, ionization, and radiation pressure. We discuss the
behavior of a large family of these models, their velocity fields, mass fluxes
and temperature, as functions of the torus properties and X-ray flux. Synthetic
warm absorber spectra are calculated, assuming pure absorption, for sample
models at various inclination angles and observing times. We show that these
models have mass fluxes and flow speeds which are comparable to those which
have been inferred from observations of Seyfert 1 warm absorbers, and that they
can produce rich absorption line spectra.Comment: 32 pages, 11 figures Accepted for publication in Ap
Neutralizing antibody response during acute and chronic hepatitis C virus infection
Little is known about the role of Abs in determining the outcome of hepatitis C virus (HCV) infection. By using infectious retroviral pseudotypes bearing HCV glycoproteins, we measured neutralizing Ab (nAb) responses during acute and chronic HCV infection. In seven acutely infected health care workers, only two developed a nAb response that failed to associate with viral clearance. In contrast, the majority of chronically infected patients had nAbs. To determine the kinetics of strain-specific and crossreactive nAb emergence, we studied patient H, the source of the prototype genotype 1a H77 HCV strain. An early weak nAb response, specific for the autologous virus, was detected at seroconversion. However, neutralization of heterologous viruses was detected only between 33 and 111 weeks of infection. We also examined the development of nAbs in 10 chimpanzees infected with H77 clonal virus. No nAb responses were detected in three animals that cleared virus, whereas strain-specific nAbs were detected in six of the seven chronically infected animals after approximately 50 weeks of infection. The delayed appearance of high titer crossreactive nAbs in chronically infected patients suggests that selective mechanism(s) may operate to prevent the appearance of these Abs during acute infection. The long-term persistence of these nAbs in chronically infected patients may regulate viral replication
An Eccentric Circumbinary Accretion Disk and the Detection of Binary Massive Black Holes
We present a two-dimensional grid-based hydrodynamic simulation of a thin,
viscous, locally-isothermal corotating disk orbiting an equal-mass Newtonian
binary point mass on a fixed circular orbit. We study the structure of the disk
after multiple viscous times. The binary maintains a central hole in the
viscously-relaxed disk with radius equal to about twice the binary semimajor
axis. Disk surface density within the hole is reduced by orders of magnitude
relative to the density in the disk bulk. The inner truncation of the disk
resembles the clearing of a gap in a protoplanetary disk. An initially circular
disk becomes elliptical and then eccentric. Disturbances in the disk contain a
component that is stationary in the rotating frame in which the binary is at
rest; this component is a two-armed spiral density wave. We measure the
distribution of the binary torque in the disk and find that the strongest
positive torque is exerted inside the central low-density hole. We make
connection with the linear theory of disk forcing at outer Lindblad resonances
(OLRs) and find that the measured torque density distribution is consistent
with forcing at the 3:2 (m=2) OLR, well within the central hole. We also
measure the time dependence of the rate at which gas accretes across the hole
and find quasi-periodic structure. We discuss implications for variability and
detection of active galactic nuclei containing a binary massive black hole.Comment: 10 pages; replaced to match ApJ version; includes new physical
interpretation of torque density (Sec. 4.1); large mpeg animation is
available at http://www.tapir.caltech.edu/~milos/circBinaryEccDisk.mp
Simulations of Direct Collisions of Gas Clouds with the Central Black Hole
We perform numerical simulations of clouds in the Galactic Centre (GC)
engulfing the nuclear super-massive black hole and show that this mechanism
leads to the formation of gaseous accretion discs with properties that are
similar to the expected gaseous progenitor discs that fragmented into the
observed stellar disc in the GC. As soon as the cloud hits the black hole, gas
with opposite angular momentum relative to the black hole collides downstream.
This process leads to redistribution of angular momentum and dissipation of
kinetic energy, resulting in a compact gaseous accretion disc. A parameter
study using thirteen high resolution simulations of homogeneous clouds falling
onto the black hole and engulfing it in parts demonstrates that this mechanism
is able to produce gaseous accretion discs that could potentially be the
progenitor of the observed stellar disc in the GC. A comparison of simulations
with different equations of state (adiabatic, isothermal and full cooling)
demonstrates the importance of including a detailed thermodynamical
description. However the simple isothermal approach already yields good results
on the radial mass transfer and accretion rates, as well as disc eccentricities
and sizes. We find that the cloud impact parameter strongly influences the
accretion rate whereas the impact velocity has a small affect on the accretion
rate.Comment: 21 pages, 18 figures, Accepted for publication in MNRA
21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies
The clumpy maser disks observed in some galactic nuclei mark the outskirts of
the accretion disk that fuels the central black hole and provide a potential
site of nuclear star formation. Unfortunately, most of the gas in maser disks
is currently not being probed; large maser gains favor paths that are
characterized by a small velocity gradient and require rare edge-on
orientations of the disk. Here we propose a method for mapping the atomic
hydrogen distribution in nuclear disks through its 21cm absorption against the
radio continuum glow around the central black hole. In NGC 4258, the 21cm
optical depth may approach unity for high angular-resolution (VLBI) imaging of
coherent clumps which are dominated by thermal broadening and have the column
density inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cm
absorption over the full rotation velocity width of the material in front of
the narrow radio jets gives a mean optical depth of ~0.1. Spectroscopic
searches for the 21cm absorption feature in other galaxies can be used to
identify the large population of inclined gaseous disks which are not masing in
our direction. Follow-up imaging of 21cm silhouettes of accelerating clumps
within these disks can in turn be used to measure cosmological distances.Comment: 4 page
The Milky Way's Fermi Bubbles: Echoes of the Last Quasar Outburst?
{\it Fermi}-LAT has recently detected two gamma ray bubbles disposed
symmetrically with respect to the Galactic plane. The bubbles have been
suggested to be in a quasi-steady state, inflated by ongoing star formation
over the age of the Galaxy. Here we propose an alternative picture where the
bubbles are the remnants of a large-scale wide-angle outflow from \sgra, the
SMBH of our Galaxy. Such an outflow would be a natural consequence of a short
but bright accretion event on to \sgra\ if it happened concurrently with the
well known star formation event in the inner 0.5 pc of the Milky Way
Myr ago. We find that the hypothesised near-spherical outflow is focussed into
a pair of symmetrical lobes by the greater gas pressure along the Galactic
plane. The outflow shocks against the interstellar gas in the Galaxy bulge.
Gamma--ray emission could be powered by cosmic rays created by either \sgra\
directly or accelerated in the shocks with the external medium. The Galaxy disc
remains unaffected, agreeing with recent observational evidence that
supermassive black holes do not correlate with galaxy disc properties. We
estimate that an accreted mass \sim 2 \times 10^3\msun is needed for the
accretion event to power the observed {\it Fermi}--LAT lobes. Within a factor
of a few this agrees with the mass of the young stars born during the star
formation event. This estimate suggests that roughly 50% of the gas was turned
into stars, while the rest accreted onto \sgra. One interpretation of this is a
reduced star formation efficiency inside the \sgra\ accretion disc due to
stellar feedback, and the other a peculiar mass deposition geometry that
resulted in a significant amount of gas falling directly inside the inner pc of the Galaxy.Comment: 6 pages, 0 figures; accepted for publication in MNRA
Star formation in self-gravitating disks in active galactic nuclei. I. Metallicity gradients in broad line regions
It has been suggested that the high metallicity generally observed in active
galactic nuclei (AGNs) and quasars originates from ongoing star formation in
the self-gravitating part of accretion disks around the supermassive black
holes. We designate this region as the star forming (SF) disk, in which metals
are produced from supernova explosions (SNexp) while at the same time inflows
are driven by SNexp-excited turbulent viscosity to accrete onto the SMBHs. In
this paper, an equation of metallicity governed by SNexp and radial advection
is established to describe the metal distribution and evolution in the SF disk.
We find that the metal abundance is enriched at different rates at different
positions in the disk, and that a metallicity gradient is set up that evolves
for steady-state AGNs. Metallicity as an integrated physical parameter can be
used as a probe of the SF disk age during one episode of SMBH activity. In the
SF disk, evaporation of molecular clouds heated by SNexp blast waves
unavoidably forms hot gas. This heating is eventually balanced by the cooling
of the hot gas, but we show that the hot gas will escape from the SF disk
before being cooled, and diffuse into the BLRs forming with a typical rate of
\sim 1\sunmyr. The diffusion of hot gas from a SF disk depends on ongoing
star formation, leading to the metallicity gradients in BLR observed in AGNs.
We discuss this and other observable consequences of this scenario.Comment: 11 pages, 5 Figures, ApJ, Vol. 737, in pres
A combined model for the X-ray to gamma-ray emission of Cyg X-1
We use recent data obtained by three (OSSE, BATSE, and COMPTEL) of four
instruments on board the Compton Gamma Ray Observatory, to construct a model of
Cyg X-1 which describes its emission in a broad energy range from soft X-rays
to MeV gamma-rays self-consistently. The gamma-ray emission is interpreted to
be the result of Comptonization, bremsstrahlung, and positron annihilation in a
hot optically thin and spatially extended region surrounding the whole
accretion disk. For the X-ray emission a standard corona-disk model is applied.
We show that the Cyg X-1 spectrum accumulated by the CGRO instruments during a
~4 year time period between 1991 and 1995, as well as the HEAO-3 gamma1 and
gamma2 spectra can be well represented by our model. The derived parameters
match the observational results obtained from X-ray measurements.Comment: 11 pages including 6 ps-figures and 2 tables, latex2e, uses
emulateapj.sty (ver. of 18 Sep 96, enclosed), epsfig.sty, times.sty. To
appear in July 20, 1998 issue of ApJ (v.502
HERPESVIRUS INFECTIONS AND CLINICAL-IMMUNOLOGIC INTERACTIONS IN PATIENTS WITH EARLY-ONSET ALZHEIMER’S DISEASE AND LATE-ONSET ALZHEIMER’S DISEASE
Alzheimer’s disease (AD) is currently the most common cause of dementia. A significant role in the pathogenesis of AD belongs to the activation of the mechanisms of neuroinflammation. There is a hypothesis that chronic infections may play a role in the maintenance of the inflammatory response in AD. The aim of this work was to study the detection rate and DNA level of herpesviruses, as well as their possible relationship with the level of the key cytokines and with clinical parameters of AD in patients with early and late onset. 30 patients with AD and 33 healthy volunteers were enrolled. The quantitative determination of DNA of CMV, EBV, HHV-6, HHV-7 was carried out by PCR. The level of cytokines and soluble IL-1β antagonist (IL-1ra) in the blood was determined by ELISA. Herpesvirus infection with increased viral load was determined if at least one of the criteria was present: 1) DNA level of EBV and/or HHV-6 > 10,000 copies/ml in saliva; 2) presence of DNA of at least one of the EBV, HHV-6, HHV-7 viruses in the blood. In the subgroup of patients with early onset and increased viral load, there was a higher increase in the levels of a number of cytokines: proinflammatory IL-8 and IL-12, a Th2-cytokine IL-4, a cytokine of the adaptive immune response IL-2. However, the level of the anti-inflammatory protein IL-1ra was lower than in the controls. These changes may indicate a dysregulation of the antiviral response, with a predominance of activation of systemic inflammation and Th2-mediated reactions. Also, in early onset AD the increased viral load was associated with lower scores on Boston naming test. The results indicate that in studies of AD mechanisms and in the search for prognostic markers of the disease, it is important to take into account the heterogeneity of AD in terms of genetic predisposition factors, risk factors, immune parameters and clinical data. Such approach is necessary for the subsequent development of personalized approaches to the prevention and treatment of AD
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