346 research outputs found
MODELING THE INFRARED EMISSION FROM DUST IN ACTIVE GALACTIC NUCLEI
Active Galactic Nuclei (AGN) are compact regions in the centers of some galaxies. They emit significantly in the whole range of the electromagnetic spectrum and show variability at different timescales. Observational evidence suggests the presence of a dusty torus obscuring the central radiation source of AGN. According to the Unified Model the observed general properties of AGN emission can be understood on the basis of orientation of this torus toward an observer. Two main types of AGN are distinguished: Type 1, with detected emission from the inner torus cavity viewed pole-on, and Type 2, viewed through the obscuring torus. There are numerous attempts in the past decade to model the emission from the torus, considering a homogeneous distribution of dust. However, important problems in explaining the observations still remain unsolved: it is hard to suppress the 10 m emission feature of silicate dust for a pole-on view and at the same time produce an absorption feature for an edge-on viewed torus; despite the huge optical depths inferred from X-ray observations of Type 2 sources, the observed absorption feature is shallow. Unlike observations, models of homogeneous tori with large optical depths always produce deep absorption feature. While it is realized that dust contained in clumps would resolve these issues, modeling of a clumpy medium poses a serious computational challenge. We are the first to incorporate clumps in our model of a dusty torus and to successfully explain the infrared emission from AGN. We model two types of clouds: directly illuminated by the AGN and diffusely heated by other clouds. We calculate the emission of the first type as angle-averaged emission from a dusty slab. The second type of clouds is modeled as dusty spheres embedded in the radiation field of the directly heated clouds. The radiative transfer problem for a dusty slab and externally heated sphere is solved exactly with our code DUSTY. The overall emission of the torus is found by integration over the spatial distribution of clouds. We find a very good agreement of our model results with observations. Comparison with them can constrain the physical conditions in the AGN dusty tori
Nuclear X-ray properties of the peculiar radio-loud hidden AGN 4C+29.30
We present results from a study of a nuclear emission of a nearby radio
galaxy, 4C+29.30, over a broad 0.5-200 keV X-ray band. This study used new
XMM-Newton (~17 ksec) and Chandra (~300 ksec) data, and archival Swift/BAT data
from the 58-month catalog. The hard (>2 keV) X-ray spectrum of 4C+29.30 can be
decomposed into an intrinsic hard power-law (Gamma ~ 1.56) modified by a cold
absorber with an intrinsic column density N_{H,z} ~ 5x10^{23} cm^{-2}, and its
reflection (|Omega/2pi| ~ 0.3) from a neutral matter including a narrow iron
Kalpha emission line at the rest frame energy ~6.4 keV. The reflected component
is less absorbed than the intrinsic one with an upper limit on the absorbing
column of N^{refl}_{H,z} < 2.5x10^{22} cm^{-2}. The X-ray spectrum varied
between the XMM-Newton and Chandra observations. We show that a scenario
invoking variations of the normalization of the power-law is favored over a
model with variable intrinsic column density. X-rays in the 0.5-2 keV band are
dominated by diffuse emission modeled with a thermal bremsstrahlung component
with temperature ~0.7 keV, and contain only a marginal contribution from the
scattered power-law component. We hypothesize that 4C+29.30 belongs to a class
of `hidden' AGN containing a geometrically thick torus. However, unlike the
majority of them, 4C+29.30 is radio-loud. Correlations between the scattering
fraction and Eddington luminosity ratio, and the one between black hole mass
and stellar velocity dispersion, imply that 4C+29.30 hosts a black hole with
~10^8 M_{Sun} mass.Comment: 13 pages, 5 figures, ApJ in pres
The Origin of the Silicate Emission Features in the Seyfert 2 Galaxy, NGC 2110
The unified model of active galactic nuclei (AGN) predicts silicate emission
features at 10 and 18 microns in type 1 AGN, and such features have now been
observed in objects ranging from distant QSOs to nearby LINERs. More
surprising, however, is the detection of silicate emission in a few type 2 AGN.
By combining Gemini and Spitzer mid-infrared imaging and spectroscopy of NGC
2110, the closest known Seyfert 2 galaxy with silicate emission features, we
can constrain the location of the silicate emitting region to within 32 pc of
the nucleus. This is the strongest constraint yet on the size of the silicate
emitting region in a Seyfert galaxy of any type. While this result is
consistent with a narrow line region origin for the emission, comparison with
clumpy torus models demonstrates that emission from an edge-on torus can also
explain the silicate emission features and 2-20 micron spectral energy
distribution of this object. In many of the best-fitting models the torus has
only a small number of clouds along the line of sight, and does not extend far
above the equatorial plane. Extended silicate-emitting regions may well be
present in AGN, but this work establishes that emission from the torus itself
is also a viable option for the origin of silicate emission features in active
galaxies of both type 1 and type 2.Comment: ApJL, accepte
Luminosity-variation independent location of the circum-nuclear, hot dust in NGC 4151
After recent sensitivity upgrades at the Keck Interferometer (KI), systematic
interferometric 2um studies of the innermost dust in nearby Seyfert nuclei are
within observational reach. Here, we present the analysis of new
interferometric data of NGC 4151, discussed in context of the results from
recent dust reverberation, spectro-photometric and interferometric campaigns.
The complete data set gives a complex picture, in particular the measured
visibilities from now three different nights appear to be rather insensitive to
the variation of the nuclear luminosity. KI data alone indicate two scenarios:
the K-band emission is either dominated to ~90% by size scales smaller than
30mpc, which falls short of any dust reverberation measurement in NGC 4151 and
of theoretical models of circum-nuclear dust distributions. Or contrary, and
more likely, the K-band continuum emission is dominated by hot dust (>= 1300K)
at linear scales of about 50mpc. The linear size estimate varies by a few tens
of percent depending on the exact morphology observed. Our interferometric,
deprojected centro-nuclear dust radius estimate of 55+-5mpc is roughly
consistent with the earlier published expectations from circum-nuclear, dusty
radiative transfer models, and spectro-photometric modeling. However, our data
do not support the notion that the dust emission size scale follows the nuclear
variability of NGC 4151 as a R_dust \propto L_nuc^0.5 scaling relation. Instead
variable nuclear activity, lagging, and variable dust response to illumination
changes need to be combined to explain the observations.Comment: 19 pages, 3 figures, 3 tables, accepted for publication in Ap
Dust Emission from Active Galactic Nuclei
Unified schemes of active galactic nuclei (AGN) require an obscuring dusty
torus around the central source, giving rise to Seyfert 1 line spectrum for
pole-on viewing and Seyfert 2 characteristics in edge-on sources. Although the
observed IR is in broad agreement with this scheme, the behavior of the 10
micron silicate feature and the width of the far-IR emission peak remained
serious problems in all previous modeling efforts. We show that these problems
find a natural explanation if the dust is contained in about 5-10 clouds along
radial rays through the torus. The spectral energy distributions (SED) of both
type 1 and type 2 sources are properly reproduced from different viewpoints of
the same object if the visual optical depth of each cloud is larger than about
60 and the clouds' mean free path increases roughly in proportion to radial
distance.Comment: 11 pages, submitted to ApJ Letter
Gas dynamics of the central few parsec region of NGC 1068 fuelled by the evolving nuclear star cluster
High resolution observations with the NIR adaptive optics integral field
spectrograph SINFONI at the VLT proved the existence of massive and young
nuclear star clusters in the centres of a sample of Seyfert galaxies. With the
help of three-dimensional high resolution hydrodynamical simulations with the
Pluto code, we follow the evolution of such clusters, focusing on stellar mass
loss. This leads to clumpy or filamentary inflow of gas on large scales (tens
of parsec), whereas a turbulent and very dense disc builds up on the parsec
scale. In order to capture the relevant physics in the inner region, we treat
this disc separately by viscously evolving the radial surface density
distribution. This enables us to link the tens of parsec scale region
(accessible via SINFONI observations) to the (sub-)parsec scale region
(observable with the MIDI instrument and via water maser emission). In this
work, we concentrate on the effects of a parametrised turbulent viscosity to
generate angular momentum and mass transfer in the disc and additionally take
star formation into account. Input parameters are constrained by observations
of the nearby Seyfert 2 galaxy NGC 1068. At the current age of its nuclear
starburst of 250 Myr, our simulations yield disc sizes of the order of 0.8 to
0.9 pc, gas masses of 1.0e6 solar masses and mass transfer rates of 0.025 solar
masses per year through the inner rim of the disc. This shows that our large
scale torus model is able to approximately account for the disc size as
inferred from interferometric observations in the mid-infrared and compares
well to the extent and mass of a rotating disc structure as inferred from water
maser observations. Several other observational constraints are discussed as
well.Comment: 13 pages, 11 figures, accepted for publication in MNRAS, updated
author list and reference
AGN Dusty Tori: II. Observational Implications of Clumpiness
From extensive radiative transfer calculations we find that clumpy torus
models with \No \about 5--15 dusty clouds along radial equatorial rays
successfully explain AGN infrared observations. The dust has standard Galactic
composition, with individual cloud optical depth \tV \about 30--100 at visual.
The models naturally explain the observed behavior of the 10\mic silicate
feature, in particular the lack of deep absorption features in AGN of any type.
The weak 10\mic emission feature tentatively detected in type 2 QSO can be
reproduced if in these sources \No drops to \about 2 or \tV exceeds \about 100.
The clouds angular distribution must have a soft-edge, e.g., Gaussian profile,
the radial distribution should decrease as or . Compact tori can
explain all observations, in agreement with the recent interferometric evidence
that the ratio of the torus outer to inner radius is perhaps as small as \about
5--10. Clumpy torus models can produce nearly isotropic IR emission together
with highly anisotropic obscuration, as required by observations. In contrast
with strict variants of unification schemes where the viewing-angle uniquely
determines the classification of an AGN into type 1 or 2, clumpiness implies
that it is only a probabilistic effect; a source can display type 1 properties
even from directions close to the equatorial plane. The fraction of obscured
sources depends not only on the torus angular thickness but also on the cloud
number \No. The observed decrease of this fraction at increasing luminosity can
be explained with a decrease of either torus angular thickness or cloud number,
but only the latter option explains also the possible emergence of a 10\mic
emission feature in QSO2.Comment: To appear in ApJ September 20, 200
The Meaning of \u3cem\u3eWISE\u3c/em\u3e Colours – I. The Galaxy and its Satellites
Through matches with the Sloan Digital Sky Survey (SDSS) catalogue we identify the location of various families of astronomical objects in WISE colour space. We identify reliable indicators that separate Galactic/local from extragalactic sources and concentrate here on the objects in our Galaxy and its closest satellites. We develop colour and magnitude criteria that are based only on WISE data to select asymptotic giant branch (AGB) stars with circumstellar dust shells, and separate them into O-rich and C-rich classes. With these criteria we produce an all-sky map for the count ratio of the two populations. The map reveals differences between the Galactic disc, the Magellanic Clouds and the Sgr Dwarf Spheroidal galaxy, as well as a radial gradient in the Large Magellanic Cloud (LMC) disc. We find that the C:O number ratio for dusty AGB stars increases with distance from the LMC centre about twice as fast as measured for near-IR selected samples of early AGB stars. Detailed radiative transfer models show that WISE colours are well explained by the emission of centrally heated dusty shells where the dust has standard properties of interstellar medium (ISM) grains. The segregation of different classes of objects in WISE colour space arises from differences in properties of the dust shells: those around young stellar objects have uniform density distributions while in evolved stars they have steep radial profiles
Isotropic Mid-Infrared Emission from the Central 100 pc of Active Galaxies
Dust reprocesses the intrinsic radiation of active galactic nuclei (AGNs) to
emerge at longer wavelengths. The observed mid-infrared (MIR) luminosity
depends fundamentally on the luminosity of the central engine, but in detail it
also depends on the geometric distribution of the surrounding dust. To quantify
this relationship, we observe nearby normal AGNs in the MIR to achieve spatial
resolution better than 100 pc, and we use absorption-corrected X-ray luminosity
as a proxy for the intrinsic AGN emission. We find no significant difference
between optically classified Seyfert 1 and 2 galaxies. Spectroscopic
differences, both at optical and IR wavelengths, indicate that the immediate
surroundings of AGNs is not spherically symmetric, as in standard unified AGN
models. A quantitative analysis of clumpy torus radiative transfer models shows
that a clumpy local environment can account for this dependence on viewing
geometry while producing MIR continuum emission that remains nearly isotropic,
as we observe, although the material is not optically thin at these
wavelengths. We find some luminosity dependence on the X-ray/MIR correlation in
the smallest scale measurements, which may indicate enhanced dust emission
associated with star formation, even on these sub-100 pc scales.Comment: 10 pages, 5 figures; accepted for publication in Ap
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