71 research outputs found
3D Radiation hydrodynamics of a dynamical torus
We have developed a new dynamical model of the torus region in active
galactic nucleus (AGN), using a three-dimensional radiation hydrodynamics
algorithm. These new simulations have the specific aim to explore the role of
radiatively-driven outflows, which is hotly debated in current literature as a
possible explanation for the observed infrared emission from the polar regions
of AGN. In this first paper, we only consider radiative effects induced by the
primary radiation from the AGN. The simulations generate a disk & outflow
structure that qualitatively agrees with observations, although the outflow is
radial rather than polar, likely due to the lack of radiation pressure from hot
dust. We find cut-offs between the wind and disk at gas temperatures of 1000 K
and dust temperatures of 100 K, producing kinematic signatures that can be used
for interpretation of high resolution infrared observations. We also produce
line emission maps to aid in the interpretation of recent ALMA observations and
future JWST observations. We investigate a number of simulation parameters, and
find that the anisotropy of the radiation field is equally important to the
Eddington factor, despite the anisotropy often being assumed to have a single
sometimes arbitrary form in many previous works. We also find that supernovae
can have a small but significant impact, but only at extremely high star
formation rates.Comment: 2nd revision, Accepted in Ap
Clumpy Dust Tori in Active Galactic Nuclei
Active Galactic Nuclei (AGN) are amongst the most luminous objects in the universe. The source of their activity is accretion onto a supermassive black hole in the center of the galactic nucleus. The various phenomena observed in AGN are explained in a common unification scheme. The cornerstone of this unification scheme of AGN is the presence of an optically and geometrically thick dust torus which surrounds the central accretion disk and broad-line region (BLR). This parsec-scaled torus is responsible for the apparent difference between type 1 and type 2 AGN. If the line-of-sight intersects with the torus, the accretion disk and BLR are not visible and the AGN is classified as a type 2 object. On the other hand, if the torus is seen nearly face-on, the accretion disk and BLR are directly exposed to the observer, so that the galaxy appears as a type 1 AGN. Near- (NIR) and mid-infrared (MIR) interferometry has resolved, for the first time, the dust torus around the nearby prototypical Seyfert 2 AGN NGC 1068. These observations provided an insight into the structure of the torus: Apparently, the dust is not smoothly distributed in the torus but arranged in clumps -- contrary to what has been commonly used in models. We developed a new radiative transfer model of clumpy dust tori which is a key tool to interpret NIR and MIR observations of AGN. The model accounts for the 3-dimensional arrangement of dust clouds. Model SEDs and images can be obtained for a number of different physical parameters (e.g., radial and vertical dust density distribution, cloud radii, optical depths, etc.). It was shown that the model SEDs are in agreement with observed spectral properties. Moreover, we applied our new model to the data of NGC 1068. It was possible, for the first time, to simultaneously reproduce NIR and MIR interferometry and photometry of the nucleus of NGC 1068. In particular, the model follows the trend of the deeper 9.7 micron silicate absorption features in the correlated fluxes than in the total fluxes, as observed with VLTI/MIDI in the 8-13 micron band. Comparison with the NGC 1068 multi-wavelength SED from Radio to the infrared shows that most of the unresolved MIR flux comes from thermal dust emission inside the torus, while in the NIR a possible synchrotron source or the accretion disk might be seen through "holes" in the clumpy torus. To get a better idea how much the accretion disk contributes to the NIR emission of AGN, we studied NIR colors of a sample of type 1 AGN which were observed in J-, H-, and K-band with HST/NICMOS. By comparing the observed colors with those expected from torus models, we found out that the accretion disk contributes typically We studied the feedback of AGN radiation on the dust torus. It was found out that dust which is smoothly distributed cannot withstand the radiation pressure from the AGN. On the other hand, self-gravitating clouds in clumpy tori can efficiently compensate the AGN radiation pressure. A physically-motivated clumpy torus model was used to study the impact of the AGN radiation on obscuration properties of the torus. We showed that below an AGN luminosity of ~10^42 erg/s, the associated low accretion rates can no longer support an obscuring torus. In the high-luminosity regime, large clouds become unbound so that the torus is dominated by smaller clouds. As a result, the covering factor and apparent scale height decrease with luminosity, so that the fraction of type 1 AGN should become larger at higher luminosities (and high radiative efficiencies). This picture offers a physical explanation for the long-standing "receding torus" phenomenon. One of the major astronomical discoveries within the last year was the identification of type 2 counterparts of QSOs. These objects were the "missing link" in the unification scheme. We studied restframe optical-to-MIR SEDs of a sample of 21 obscured QSOs with our clumpy torus model. It was found out that the observed SEDs favor models with compact geometries and, apparently, no flaring. In some objects, the combination of blue NIR color and very deep silicate absorption is in contradiction to expectations from torus models. We propose that in such cases, the torus is actually seen face-on, and a detached cold absorber in the host galaxy (e.g., a dust lane or cloud) is responsible for the deep silicate absorption feature. According to this picture, some of the obscured QSOs are mimicking type 2 AGN although their torus orientation might be similar to a type 1 AGN
Differential interferometry of QSO broad line regions I: improving the reverberation mapping model fits and black hole mass estimates
Reverberation mapping estimates the size and kinematics of broad line regions
(BLR) in Quasars and type I AGNs. It yields size-luminosity relation, to make
QSOs standard cosmological candles, and mass-luminosity relation to study the
evolution of black holes and galaxies. The accuracy of these relations is
limited by the unknown geometry of the BLR clouds distribution and velocities.
We analyze the independent BLR structure constraints given by super-resolving
differential interferometry. We developed a three-dimensional BLR model to
compute all differential interferometry and reverberation mapping signals. We
extrapolate realistic noises from our successful observations of the QSO 3C273
with AMBER on the VLTI. These signals and noises quantify the differential
interferometry capacity to discriminate and measure BLR parameters including
angular size, thickness, spatial distribution of clouds, local-to-global and
radial-to-rotation velocity ratios, and finally central black hole mass and BLR
distance. A Markov Chain Monte Carlo model-fit, of data simulated for various
VLTI instruments, gives mass accuracies between 0.06 and 0.13 dex, to be
compared to 0.44 dex for reverberation mapping mass-luminosity fits. We
evaluate the number of QSOs accessible to measures with current (AMBER),
upcoming (GRAVITY) and possible (OASIS with new generation fringe trackers)
VLTI instruments. With available technology, the VLTI could resolve more than
60 BLRs, with a luminosity range larger than four decades, sufficient for a
good calibration of RM mass-luminosity laws, from an analysis of the variation
of BLR parameters with luminosity.Comment: 19 pages, 14 figures, accepted by MNRAS on December 5, 201
A diversity of dusty AGN tori: Data release for the VLTI/MIDI AGN Large Program and first results for 23 galaxies
The AGN-heated dust distribution (the "torus") is increasingly recognized not
only as the absorber required in unifying models, but as a tracer for the
reservoir that feeds the nuclear Super-Massive Black Hole. Yet, even its most
basic structural properties (such as its extent, geometry and elongation) are
unknown for all but a few archetypal objects. Since most AGNs are unresolved in
the mid-infrared, we utilize the MID-infrared interferometric Instrument (MIDI)
at the Very Large Telescope Interferometer (VLTI) that is sensitive to
structures as small as a few milli-arcseconds (mas). We present here an
extensive amount of new interferometric observations from the MIDI AGN Large
Program (2009 - 2011) and add data from the archive to give a complete view of
the existing MIDI observations of AGNs. Additionally, we have obtained
high-quality mid-infrared spectra from VLT/VISIR. We present correlated and
total flux spectra for 23 AGNs and derive flux and size estimates at 12 micron
using simple axisymmetric geometrical models. Perhaps the most surprising
result is the relatively high level of unresolved flux and its large scatter:
The median "point source fraction" is 70 % for type 1 and 47 % for type 2 AGNs
meaning that a large part of the flux is concentrated on scales smaller than
about 5 mas (0.1 - 10 pc). Among sources observed with similar spatial
resolution, it varies from 20 % - 100 %. For 18 of the sources, two nuclear
components can be distinguished in the radial fits. While these models provide
good fits to all but the brightest sources, significant elongations are
detected in eight sources. The half-light radii of the fainter sources are
smaller than expected from the size ~ L^0.5 scaling of the bright sources and
show a large scatter, especially when compared to the relatively tight
size--luminosity relation in the near-infrared.Comment: A&A in press; 93 pages, 63 figures, 39 tables; data available only
via CD
A dust-parallax distance of 19 megaparsecs to the supermassive black hole in NGC 4151
The active galaxy NGC 4151 has a crucial role as one of only two active
galactic nuclei for which black hole mass measurements based on emission line
reverberation mapping can be calibrated against other dynamical methods.
Unfortunately, effective calibration requires an accurate distance to NGC 4151,
which is currently not available. Recently reported distances range from 4 to
29 megaparsecs (Mpc). Strong peculiar motions make a redshift-based distance
very uncertain, and the geometry of the galaxy and its nucleus prohibit
accurate measurements using other techniques. Here we report a dust-parallax
distance to NGC 4151 of Mpc. The measurement is
based on an adaptation of a geometric method proposed previously using the
emission line regions of active galaxies. Since this region is too small for
current imaging capabilities, we use instead the ratio of the
physical-to-angular sizes of the more extended hot dust emission as determined
from time-delays and infrared interferometry. This new distance leads to an
approximately 1.4-fold increase in the dynamical black hole mass, implying a
corresponding correction to emission line reverberation masses of black holes
if they are calibrated against the two objects with additional dynamical
masses.Comment: Authors' version of a letter published in Nature (27 November 2014);
8 pages, 5 figures, 1 tabl
Resolving the AGN and host emission in the mid-infrared using a model-independent spectral decomposition
We present results on the spectral decomposition of 118 Spitzer Infrared
Spectrograph (IRS) spectra from local active galactic nuclei (AGN) using a
large set of Spitzer/IRS spectra as templates. The templates are themselves IRS
spectra from extreme cases where a single physical component (stellar,
interstellar, or AGN) completely dominates the integrated mid-infrared
emission. We show that a linear combination of one template for each physical
component reproduces the observed IRS spectra of AGN hosts with unprecedented
fidelity for a template fitting method, with no need to model extinction
separately. We use full probability distribution functions to estimate
expectation values and uncertainties for observables, and find that the
decomposition results are robust against degeneracies. Furthermore, we compare
the AGN spectra derived from the spectral decomposition with sub-arcsecond
resolution nuclear photometry and spectroscopy from ground-based observations.
We find that the AGN component derived from the decomposition closely matches
the nuclear spectrum, with a 1-sigma dispersion of 0.12 dex in luminosity and
typical uncertainties of ~0.19 in the spectral index and ~0.1 in the silicate
strength. We conclude that the emission from the host galaxy can be reliably
removed from the IRS spectra of AGN. This allows for unbiased studies of the
AGN emission in intermediate and high redshift galaxies -currently inaccesible
to ground-based observations- with archival Spitzer/IRS data and in the future
with the Mid-InfraRed Instrument of the James Webb Space Telescope. The
decomposition code and templates are available at
http://www.denebola.org/ahc/deblendIRS.Comment: 16 pages, 15 figures, 2 tables, accepted for publication in Ap
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