3,651 research outputs found
On the dust tori in Palomar-Green quasars
The dust clouds in the torus of the quasar are irradiated by the central
source, and the clouds at the inner radius of the torus re-radiate mostly in
the near-infrared (NIR) wavebands. The ratio of the near-infrared luminosity to
the bolometric luminosity L_NIR/L_bol can therefore reflect the torus geometry
to some extent. We find a significant correlation between the ratio of the
near-infrared luminosity to the bolometric luminosity L_NIR/L_bol and the
central black hole mass M_bh for the Palomar-Green(PG) quasars, whereas no
correlation is found between the Eddington ratio L_bol/L_Edd and the ratio
L_NIR/L_bol. Similar correlations are found for the mid-infrared and
far-infrared cases. It may imply that the torus geometry, i.e., the solid angle
subtended by the dust torus as seen from the central source, does not evolve
with the accretion rate. The correlation of the solid angle subtended by the
torus with the central black hole mass M_bh implies that the formation of the
dust torus is likely regulated by the central black hole mass. We find that the
torus thickness H increases with quasar bolometric luminosities, which is
different from the constant torus thickness H with luminosity assumed in the
receding torus model. The mean covering factor of the dust clouds at the inner
radius of the torus derived from the IR emission data is ~0.39 for PG quasars.
The average relative thickness H/R of the tori in the PG quasars derived from
the ratios of the infrared to bolometric luminosities is ~0.9. We suggest that
the further IR observations on a larger quasar sample including more fainter
quasars by the Spitzer Space Telescope will help understand the physics of the
dust tori in quasars.Comment: The incorrect V-magnitude used for 1351+640 is fixed, the main
conclusions are not changed, accepted for publication in Ap
Extremely Luminous Water Vapor Emission from a Type 2 Quasar at Redshift z = 0.66
A search for water masers in 47 Sloan Digital Sky Survey Type 2 quasars using
the Green Bank Telescope has yielded a detection at a redshift of z = 0.660.
This maser is more than an order of magnitude higher in redshift than any
previously known and, with a total isotropic luminosity of 23,000 L_sun, also
the most powerful. The presence and detectability of water masers in quasars at
z ~ 0.3-0.8 may provide a better understanding of quasar molecular tori and
disks, as well as fundamental quasar and galaxy properties such as black hole
masses. Water masers at cosmologically interesting distances may also
eventually provide, via direct distance determinations, a new cosmological
observable for testing the reality and properties of dark energy, currently
inferred primarily through Type 1a supernova measurements.Comment: 8 pages including 1 figure; accepted for publication in ApJ Letter
3C 216: A Powerful FRII Seyfert 1 Galaxy
3C 216 has a weak accretion flow luminosity, well below the Seyfert1/QSO
dividing line, weak broad emission lines (BELs) and powerful radio lobes. As a
consequence of the extreme properties of 3C 216, it is the most convincing
example known of an FR II radio source that is kinetically dominated: the jet
kinetic luminosity, , is larger than the total thermal luminosity (IR to
X-ray) of the accretion flow, . Using three independent estimators for
the central black hole mass, we find that the jet in 3C 216 is very
super-Eddington, , where is the long
term time averaged , calculated at 151 MHz. It is argued that 3C 216
satisfies the contemporaneous kinetically dominated condition, , either presently or in the past based on the rarity of
quasars. The existence of AGN is a strong constraint
on the theory of the central engine of FRII radio sources
AGN Obscuring Tori Supported by Infrared Radiation Pressure
Explicit 2-d axisymmetric solutions are found to the hydrostatic equilibrium,
energy balance, and photon diffusion equations within obscuring tori around
active galactic nuclei. These solutions demonstrate that infrared radiation
pressure can support geometrically thick structures in AGN environments subject
to certain constraints: the bolometric luminosity must be roughly 0.03--1 times
the Eddington luminosity; and the Compton optical depth of matter in the
equatorial plane should be order unity, with a tolerance of about an order of
magnitude up or down. Both of these constraints are at least roughly consistent
with observations. In addition, angular momentum must be redistributed so that
the fractional rotational support against gravity rises from the inner edge of
the torus to the outer in a manner specific to the detailed shape of the
gravitational potential. This model also predicts that the column densities
observed in obscured AGN should range from about 10^{22} to about 10^{24}
cm^{-2}.Comment: ApJ, in pres
The Relationship of Hard X-ray and Optical Line Emission in Low Redshift Active Galactic Nuclei
In this paper we assess the relationship of the population of Active Galactic
Nuclei (AGN) selected by hard X-rays to the traditional population of AGN with
strong optical emission lines. First, we study the emission-line properties of
a new hard X-ray selected sample of 47 local AGN (classified optically as both
Type 1 and Type 2 AGN). We find that the hard X- ray (3-20 keV) and
[OIII]5007 optical emission-line luminosities are well-correlated over
a range of about four orders-of-magnitude in luminosity (mean luminosity ratio
2.15 dex with a standard deviation of = 0.51 dex). Second, we study
the hard X-ray properties of a sample of 55 local AGN selected from the
literature on the basis of the flux in the [OIII] line. The correlation between
the hard X-ray (2-10 keV) and [OIII] luminosity for the Type 1 AGN is
consistent with what is seen in the hard X-ray selected sample. However, the
Type 2 AGN have a much larger range in the luminosity ratio, and many are very
weak in hard X-rays (as expected for heavily absorbed AGN). We then compare the
hard X-ray (3-20 keV) and [OIII] luminosity functions of AGN in the local
universe. These have similar faint-end slopes with a luminosity ratio of 1.60
dex (0.55 dex smaller than the mean value for individual hard X-ray selected
AGN). We conclude that at low redshift, selection by narrow optical emission-
lines will recover most AGN selected by hard X-rays (with the exception of BL
Lac objects). However, selection by hard X-rays misses a significant fraction
of the local AGN population with strong emission lines
Measuring the Fraction of Obscured Quasars by the Infrared Luminosity of Unobscured Quasars
Recent work has suggested that the fraction of obscured AGN declines with
increasing luminosity, but it has been difficult to quantify this trend. Here,
we attempt to measure this fraction as a function of luminosity by studying the
ratio of mid-infrared to intrinsic nuclear bolometric luminosity in unobscured
AGN. Because the mid-infrared is created by dust reprocessing of shorter
wavelength nuclear light, this ratio is a diagnostic of f_obsc, the fraction of
solid angle around the nucleus covered by obscuring matter. In order to
eliminate possible redshift-dependences while also achieving a large dynamic
range in luminosity, we have collected archival 24 micron MIPS photometry from
objects with z~1 in the Sloan Digital Sky Survey (SDSS), the Great
Observatories Origins Deep Survey (GOODS) and the Cosmic Evolution Survey
(COSMOS). To measure the bolometric luminosity for each object, we used
archival optical data supplemented by GALEX data. We find that the mean ratio
of 24 microns to bolometric luminosity decreases by a factor of ~3 in the
L_bol=10^44-3x10^47 ergs s^-1 range, but there is also a large scatter at
constant L_bol. Using radiation transfer solutions for model geometries, we
show how the IR/bolometric ratio relates to f_obsc and compare these values
with those obtained obtained from samples of X-ray selected AGN. Although we
find approximate agreement, our method indicates somewhat higher values of
f_obsc, particularly in the middle range of luminosities, suggesting that there
may be a significant number of heavily obscured AGN missed by X-ray surveys.Comment: ApJ, in press. 10 pages in emulateapj style, 4 figures, 3 table
Further insights on predictors of environmental tobacco smoke exposure during the pediatric age
Background: The smoking ban in public places has reduced Environmental Tobacco Smoke (ETS) exposure for non-smokers, but despite this, domestic environments still remain places at high risk of exposure, and, today, about 40% of children worldwide are exposed to ETS at home. The aims of the study are to investigate the contribution of several factors on ETS exposure among a group of Italian children and to evaluate the changes in smoking precautions adopted at home when the smoker is the mother, the father, or both parents, respectively. Methods: A cross-sectional study was performed on a sample of 519 Italian schoolchildren. Information was collected via a questionnaire. Results: 41.4% of the participants lived with at least one smoker. Almost half of the children exposed to ETS lived with one or more smokers who do not observe any home smoking ban. Lower maternal or paternal educational levels significantly increase the risk of ETS exposure at home and the âworst caseâ is represented by both parents who smoke. Conclusions: More effective preventive interventions are needed to protect children from ETS exposure. Some interventions should be specifically dedicated to smokers with a low educational level and to mothers that smoke
Identifying Compact Symmetric Objects in the Southern Sky
We present results of multifrequency polarimetric VLBA observations of 20
compact radio sources. The observations represent the northern and southern
extensions of a large survey undertaken to identify Compact Symmetric Objects
(CSOs) Observed in the Northern Sky (COINS). CSOs are young radio galaxies
whose jet axes lie close to the plane of the sky, and whose appearance is
therefore not dominated by relativistic beaming effects. The small linear sizes
of CSOs make them valuable for studies of both the evolution of radio galaxies
and testing unified schemes for active galactic nuclei (AGN). In this paper we
report on observations made of 20 new CSO candidates discovered in the northern
and southern extremities of the VLBA Calibrator Survey. We identify 4 new CSOs,
and discard 12 core-jet sources. The remaining 4 sources remain candidates
pending further investigation. We present continuum images at 5 GHz and 15 GHz
and, where relevant, images of the polarized flux density and spectral index
distributions for the 8 new CSOs and CSO candidates.Comment: accepted to Ap
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