1,658 research outputs found
VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068
We have made the first VLBI synthesis images of the H2O maser emission
associated with the central engine of the Seyfert galaxy NGC 1068. Emission
extends about +/-300 km/s from the systemic velocity. Images with
submilliarcsecond angular resolution show that the red-shifted emission lies
along an arc to the northwest of the systemic emission. (The blue-shifted
emission has not yet been imaged with VLBI.) Based on the maser velocities and
the relative orientation of the known radio jet, we propose that the maser
emission arises on the surface of a nearly edge-on torus, where physical
conditions are conducive to maser action. The visible part of the torus is
axially thick, with comparable height and radius. The velocity field indicates
sub-Keplerian differential rotation around a central mass of about 1e7 Msun
that lies within a cylindrical radius of about 0.65 pc. The estimated
luminosity of the central engine is about 0.5 of the Eddington limit. There is
no detectable compact radio continuum emission near the proposed center of the
torus (T_B< 5e6 K on size scales of about 0.1 pc), so that the observed
flat-spectrum core cannot be direct self-absorbed synchrotron radiation.Comment: 12 pages, 4 figures. To appear in ApJ Part 2. Also available at
http://www.physics.ucsb.edu/~vlbiweb
Spectropolarimetry of 3CR 68.1: A Highly Inclined Quasar
We present Keck spectropolarimetry of the highly polarized radio-loud quasar
3CR 68.1 (z=1.228, V=19). The polarization increases from 5 in the red (4000 A
rest-frame) to >10% in the blue (1900 A rest-frame). The broad emission lines
are polarized the same as the continuum, which shows that 3CR 68.1 is not a
blazar as it has sometimes been regarded in the past. We also present
measurements of the emission lines and a strong, blueshifted, associated
absorption line system, as well as a detection at the emission-line redshift of
Ca II K absorption, presumably from stars in the host galaxy. 3CR 68.1 belongs
to an observationally rare class of highly polarized quasars that are neither
blazars nor partially obscured radio-quiet QSOs. Taking into account 3CR 68.1's
other unusual properties, such as its extremely red spectral energy
distribution and its extreme lobe dominance, we explain our spectropolarimetric
results in terms of unified models. We argue that we have a dusty, highly
inclined view of 3CR 68.1, with reddened scattered (polarized) quasar light
diluted by even more dust-reddened quasar light reaching us directly from the
nucleus.Comment: 20 pages, includes 3 tables, 6 figures. Accepted by Ap
An extreme, blueshifted iron line profile in the Narrow Line Seyfert 1 PG 1402+261; an edge-on accretion disk or highly ionized absorption?
We report on a short XMM-Newton observation of the radio-quiet Narrow Line
Seyfert 1 PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong
excess of counts between 6-9 keV in the rest frame. This feature can be modeled
by an unusually strong (equivalent width 2 keV) and very broad (FWHM velocity
of 110000 km/s) iron K-shell emission line. The line centroid energy at 7.3 keV
appears blue-shifted with respect to the iron Kalpha emission band between
6.4-6.97 keV, while the blue-wing of the line extends to 9 keV in the quasar
rest frame. The line profile can be fitted by reflection from the inner
accretion disk, but an inclination angle of >60 deg is required to model the
extreme blue-wing of the line. Furthermore the extreme strength of the line
requires a geometry whereby the hard X-ray emission from PG 1402+261 above 2
keV is dominated by the pure-reflection component from the disk, while little
or none of the direct hard power-law is observed. Alternatively the spectrum
above 2 keV may instead be explained by an ionized absorber, if the column
density is sufficiently high (N_H > 3 x 10^23 cm^-2) and if the matter is
ionized enough to produce a deep (tau~1) iron K-shell absorption edge at 9 keV.
This absorber could originate in a large column density, high velocity outflow,
perhaps similar to those which appear to be observed in several other high
accretion rate AGN. Further observations, especially at higher spectral
resolution, are required to distinguish between the accretion disk reflection
or outflow scenarios.Comment: Accepted for publication in ApJ (18 pages, 5 figures, 1 table
Optimal constants for a nonlocal approximation of sobolev norms and total variation
We consider the family of nonlocal and nonconvex functionals proposed and investigated by J. Bourgain, H. Brezis and H.-M. Nguyen in a series of papers of the last decade. It was known that this family of functionals Gamma-converges to a suitable multiple of the Sobolev norm or the total variation, depending on the summability exponent, but the exact constants and the structure of recovery families were still unknown, even in dimension 1. We prove a Gamma-convergence result with explicit values of the constants in any space dimension. We also show the existence of recovery families consisting of smooth functions with compact support. The key point is reducing the problem first to dimension 1, and then to a finite combinatorial rearrangement inequality
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
Obscuring Active Galactic Nuclei with Nuclear Starburst Disks
We assess the potential of nuclear starburst disks to obscure the
Seyfert-like AGN that dominate the hard X-ray background at z~1. Over 1200
starburst disk models, based on the theory developed by Thompson et al., are
calculated for five input parameters: the black hole mass, the radial size of
the starburst disk, the dust-to-gas ratio, the efficiency of angular momentum
transport in the disk, and the gas fraction at the outer disk radius. We find
that a large dust-to-gas ratio, a relatively small starburst disk, a
significant gas mass fraction, and efficient angular momentum transport are all
important to produce a starburst disk that can potentially obscure an AGN. The
typical maximum star-formation rate in the disks is ~10 solar masses per year.
Assuming no mass-loss due to outflows, the starburst disks feed gas onto the
black hole at rates sufficient to produce hard X-ray luminosities of
10^{43}-10^{44} erg s^{-1}. The starburst disks themselves should be detectable
at mid-infrared and radio wavelengths; at z=0.8, the predicted fluxes are ~1
mJy at 24microns and ~10-30 microJy at 1.4GHz. Thus, we predict a large
fraction of radio/X-ray matches in future deep radio surveys. The starburst
disks should be easily distinguished from AGN in future 100microns surveys by
Herschel with expected fluxes of ~5 mJy. Any AGN-obscuring starbursts will be
associated with hot dust, independent of AGN heating, resulting in observable
signatures for separating galactic and nuclear star-formation. Finally, because
of the competition between gas and star-formation, nuclear starbursts will be
associated with lower-luminosity AGN. Thus, this phenomenon is a natural
explanation for the observed decrease in the fraction of obscured AGN with
luminosity.Comment: 13 pages, 12 figures, 3 in color; accepted by Ap
Formation of Large-Scale Obscuring Wall and AGN Evolution Regulated by Circumnuclear Starbursts
By considering the radiative force by a circumnuclear starburst as well as an
AGN, we analyze the equilibrium configuration and the stability of dusty gas in
the circumnuclear regions. It is found that the radiative force by an intensive
starburst can support a stable gaseous wall with a scale-height of several
hundred parsecs. Moreover, by taking the simple stellar evolution in the
starburst into account, we find that the covering factor of the wall decreases
on a time-scale of several yr. The large-scale wall, if formed, works to
obscure the nucleus due to the dust opacity. Hence, it is anticipated that the
index of AGN type tends to shift from higher to lower in several yr
according as the circumnuclear starburst becomes dimmer. On the other hand, if
the AGN itself is brighter than the circumnuclear starburst (e.g. quasar case),
no stable large-scale wall forms. In that case, the AGN is highly probably
identified as type 1. The present mechanism may provide a physical explanation
for the putative correlation between AGN type and host properties that Sy2's
are more frequently associated with circumnuclear starbursts than Sy1's,
whereas quasars are mostly observed as type 1 regardless of star-forming
activity in the host galaxies.Comment: 10 pages, 3 figures, ApJ Letters in pres
Evidence for Quasar Activity Triggered by Galaxy Mergers in HST Observations of Dust-reddened Quasars
We present Hubble ACS images of thirteen dust reddened Type-1 quasars
selected from the FIRST/2MASS Red Quasar Survey. These quasars have high
intrinsic luminosities after correction for dust obscuration (-23.5 > M_B >
-26.2 from K-magnitude). The images show strong evidence of recent or ongoing
interaction in eleven of the thirteen cases, even before the quasar nucleus is
subtracted. None of the host galaxies are well fit by a simple elliptical
profile. The fraction of quasars showing interaction is significantly higher
than the 30% seen in samples of host galaxies of normal, unobscured quasars.
There is a weak correlation between the amount of dust reddening and the
magnitude of interaction in the host galaxy, measured using the Gini
coefficient and the Concentration index. Although few host galaxy studies of
normal quasars are matched to ours in intrinsic quasar luminosity, no evidence
has been found for a strong dependence of merger activity on host luminosity in
samples of the host galaxies of normal quasars. We thus believe that the high
merger fraction in our sample is related to their obscured nature, with a
significant amount of reddening occurring in the host galaxy. The red quasar
phenomenon seems to have an evolutionary explanation, with the young quasar
spending the early part of its lifetime enshrouded in an interacting galaxy.
This might be further indication of a link between AGN and starburst galaxies.Comment: 18 pages, 6 low resolution figures, accepted for publication in Ap
The UV Continuum of Quasars: Models and SDSS Spectral Slopes
We measure long (2200-4000 ang) and short (1450-2200 ang) wavelength spectral
slopes \alpha (F_\nu proportional to \nu^\alpha) for quasar spectra from the
Sloan Digital Sky Survey. The long and short wavelength slopes are computed
from 3646 and 2706 quasars with redshifts in the z=0.76-1.26 and z=1.67-2.07
ranges, respectively. We calculate mean slopes after binning the data by
monochromatic luminosity at 2200 ang and virial mass estimates based on
measurements of the MgII line width and 3000 ang continuum luminosity. We find
little evidence for mass dependent variations in the mean slopes, but a
significant luminosity dependent trend in the near UV spectral slopes is
observed with larger (bluer) slopes at higher luminosities. The far UV slopes
show no clear variation with luminosity and are generally lower (redder) than
the near UV slopes at comparable luminosities, suggesting a slightly concave
quasar continuum shape. We compare these results with Monte Carlo distributions
of slopes computed from models of thin accretion disks, accounting for
uncertainties in the mass estimates. The model slopes produce mass dependent
trends which are larger than observed, though this conclusion is sensitive to
the assumed uncertainties in the mass estimates. The model slopes are also
generally bluer than observed, and we argue that reddening by dust intrinsic to
the source or host galaxy may account for much of the discrepancy.Comment: To be published in ApJ, 18 pages, 10 figure
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