39 research outputs found
Investigating the structure of active galactic nuclei : the dusty torus
Active galactic nuclei Nowadays it is widely accepted that every massive galaxy harbors a supermassive black hole (SMBH) at its center. A number of apparent correlations between SMBH mass and host galaxy structural and dynamical properties have been observed. The correlation between the masses of SMBHs and their host galactic bulges suggest a link between their growth (Kormendy & Richstone, 1995; Kormendy & Gebhardt, 2001). Active galactic nucleus (AGN) represents a phase in the life of a galaxy, during which the SMBH growth is directly observable. The term AGN encompasses a variety of energetic phenomena in galactic centers triggered by the matter spiralling into a SMBH at a relatively high rate. The radiation coming from AGNs originates in the conversion of gravitational potential energy into thermal energy as matter spirals towards the SMBH through an accretion disk (Lynden- Bell, 1969). Their luminosity can be up to 10000 greater then the total luminosity of a normal galaxy. The radiated AGN continuum covers a broad range of spectrum, from the X to radio domain, it is partially polarized and variable in time. Radiation from the central engine is ionizing the surrounding medium, creating conditions for the strong emission line spectrum, superimposed on the continuum. Sometimes, highly collimated and fast outflows (“jets”) emerge perpendicular to the accretion disk. Since the discovery of Keel (1980) that the orientation of Seyfert 1 galaxies is not random, it xxx has been recognized that the appearance of an AGN varies with the viewing angle. This has led to the picture of “orientation unification” (see Antonucci, 1993; Urry & Padovani, 1995) where the structure of AGNs is believed to be basically similar but what we see is a strong function of orientation (see Fig. 9). In this unified model, the central black hole is surrounded by a geometrically-thin accretion disk which is the source of the strong X-ray emission and UV/optical continuum (see Jovanovic, 2012, and references therein). Above and below the disk is the broad-line region (BLR), turbulent, rapidly-moving, dense, emission-line gas orbiting the black hole (see Gaskell, 2009, for a review). Both the accretion disk and the BLR are surrounded by a geometrically- and optically-thick, roughly toroidal structure of dust and gas (the “dusty torus”), which is absorbing the incoming radiation and re-emitting it in the infrared (IR). In addition to these components there is lower density, more slowly moving gas present on a scale similar to or significantly larger than that of the torus. This gas can be seen when it is illuminated by the cone of ionizing radiation emanating from inside the torus. It is a source of narrow emission lines and thus is know as the “narrow-line region” (NLR). The broad emission lines and the thermal continuum emission can only be seen when the torus is close to faceon and thus, such an object appears as a type 1 active galaxy. Close to edge-on orientations, the dusty torus blocks the radiation coming from the accretion disk and BLR. In this case an UV/optical bump and broad emission lines are absent and an object appears as a type 2 active galaxy. If jet of matter, ejected perpendicular to the accretion disk is present, then viewing such an object along the jet would exhibit strong non-thermal, polarized and rapidly variable continuum. The masses of SMBHs can be readily estimated in some types of AGN, (Dibai, 1977) and AGNs are currently our only way of studying the evolution of SMBHs over cosmic time. Furthermore, the brightest AGNs are the most luminous quasi-steady compact sources of radiation in the universe and hence they are valuable probes of cosmic evolution up to very high redshifts. In order to understand black hole growth across cosmic time and the connection between galaxies and black holes, we need to understand how AGNs work. We need to test the basic picture outlined above and, in particular, to be able to explain observations which presently challenge this picture and might force modifications of it
Dissecting the active galactic nucleus in Circinus -- I. Peculiar mid-IR morphology explained by a dusty hollow cone
Recent high angular resolution observations resolved for the first time the
mid-infrared (MIR) structure of nearby active galactic nuclei (AGN).
Surprisingly, they revealed that a major fraction of their MIR emission comes
from the polar regions. This is at odds with the expectation based on AGN
unification, which postulates a dusty torus in the equatorial region. The
nearby, archetypical AGN in the Circinus galaxy offers one of the best
opportunities to study the MIR emission in greater detail. New, high quality
MIR images obtained with the upgraded VISIR instrument at the Very Large
Telescope show that the previously detected bar-like structure extends up to at
least 40 pc on both sides of the nucleus along the edges of the ionization
cone. Motivated by observations across a wide wavelength range and on different
spatial scales, we propose a phenomenological dust emission model for the AGN
in the Circinus galaxy consisting of a compact dusty disk and a large-scale
dusty cone shell, illuminated by a tilted accretion disk with an anisotropic
emission pattern. Undertaking detailed radiative transfer simulations, we
demonstrate that such a model is able to explain the peculiar MIR morphology
and account for the entire IR spectral energy distribution. Our results call
for caution when attributing dust emission of unresolved sources entirely to
the torus and warrant further investigation of the MIR emission in the polar
regions of AGN.Comment: Accepted to MNRAS. Version 2: typos correcte
The AGN dusty torus as a clumpy two-phase medium: radiative transfer modeling with SKIRT
We modeled the AGN dusty torus as a clumpy two-phase medium, with
high-density clumps embedded in a low-density interclump dust. To obtain
spectral energy distributions and images of the torus at different wavelengths,
we employed the 3D Monte Carlo radiative transfer code SKIRT. Apart from the
grid of two-phase models, we calculated the corresponding sets of clumps-only
models and models with a smooth dust distribution for comparison. We found that
the most striking feature of the two-phase model is that it might offer a
natural solution to the common issue reported in a number of papers -- the
observed excess of the near-infrared emission.Comment: Proceedings of the Torus Workshop 2012 held at the University of
Texas at San Antonio, 5-7 December 2012. C. Packham, R. Mason, and A.
Alonso-Herrero (eds.). 8 pages, 5 figures. A grid of model SEDs available at
https://sites.google.com/site/skirtorus
Gravitational microlensing of AGN dusty tori
We investigated the gravitational microlensing of active galactic nucleus
dusty tori in the case of lensed quasars in the infrared domain. The dusty
torus is modeled as a clumpy two-phase medium. To obtain spectral energy
distributions and images of tori at different wavelengths, we used the 3D Monte
Carlo radiative transfer code SKIRT. A ray-shooting technique has been used to
calculate microlensing magnification maps. We simulated microlensing by the
stars in the lens galaxy for different configurations of the lensed system and
different values of the torus parameters, in order to estimate (a) amplitudes
and timescales of high magnification events, and (b) the influence of
geometrical and physical properties of dusty tori on light curves in the
infrared domain. We found that, despite their large size, dusty tori could be
significantly affected by microlensing in some cases, especially in the
near-infrared domain (rest-frame). The very long time-scales of such events, in
the range from several decades to hundreds of years, are limiting the practical
use of this method to study the properties of dusty tori. However, our results
indicate that, when studying flux ratios between the images in different
wavebands of lensed quasars, one should not disregard the possibility that the
near and mid-infrared flux ratios could be under the influence of microlensing.Comment: 10 pages, 6 figures, 1 table, MNRAS accepted. V3: corrected values in
Table
Polarization in Monte Carlo radiative transfer and dust scattering polarization signatures of spiral galaxies
Polarization is an important tool to further the understanding of interstellar dust and the sources behind it. In this paper we describe our implementation of polarization that is due to scattering of light by spherical grains and electrons in the dust Monte Carlo radiative transfer code SKIRT. In contrast to the implementations of other Monte Carlo radiative transfer codes, ours uses co-moving reference frames that rely solely on the scattering processes. It fully supports the peel-off mechanism that is crucial for the efficient calculation of images in 3D Monte Carlo codes. We develop reproducible test cases that push the limits of our code. The results of our program are validated by comparison with analytically calculated solutions. Additionally, we compare results of our code to previously published results. We apply our method to models of dusty spiral galaxies at near-infrared and optical wavelengths. We calculate polarization degree maps and show them to contain signatures that trace characteristics of the dust arms independent of the inclination or rotation of the galaxy
AGN Dusty Tori as a Clumpy Two-Phase Medium: The 10 Micron Silicate Feature
We investigated the emission of active galactic nuclei dusty tori in the
infrared domain, with a focus on the 10 micron silicate feature. We modeled the
dusty torus as a clumpy two-phase medium with high-density clumps and a
low-density medium filling the space between the clumps. We employed a
three-dimensional radiative transfer code to obtain spectral energy
distributions and images of tori at different wavelengths. We calculated a grid
of models for different parameters and analyzed the influence of these
parameters on the shape of the mid-infrared emission. A corresponding set of
clumps-only models and models with a smooth dust distribution is calculated for
comparison. We found that the dust distribution, the optical depth and a random
arrangement of clumps in the innermost region, all have an impact on the shape
and strength of the silicate feature. The 10 micron silicate feature can be
suppressed for some parameters, but models with smooth dust distribution are
also able to produce a wide range of the silicate feature strength.Comment: 5 pages, 2 figures. Proceedings of the "8th Serbian Conference on
Spectral Line Shapes in Astrophysics", Divcibare, Serbia, June 6-10 2011.
Model SEDs available for download at https://sites.google.com/site/skirtorus
The dust covering factor in active galactic nuclei
The primary source of emission of active galactic nuclei (AGNs), the accretion disc, is surrounded by an optically and geometrically thick dusty structure ('the so-called dusty torus'). The infrared radiation emitted by the dust is nothing but a reprocessed fraction of the accretion disc emission, so the ratio of the torus to the AGN luminosity (L-torus/L-AGN) should corresponds to the fraction of the sky obscured by dust, i.e. the covering factor. We undertook a critical investigation of the L-torus/L-AGN as the dust covering factor proxy. Using state-of-the-art 3D Monte Carlo radiative transfer code, we calculated a grid of spectral energy distributions (SEDs) emitted by the clumpy two-phase dusty structure. With this grid of SEDs, we studied the relation between L-torus/L-AGN and the dust covering factor for different parameters of the torus. We found that in the case of type 1 AGNs the torus anisotropy makes L-torus/L-AGN underestimate low covering factors and overestimate high covering factors. In type 2 AGNs L-torus/L-AGN always underestimates covering factors. Our results provide a novel easy-to-use method to account for anisotropy and obtain correct covering factors. Using two samples from the literature, we demonstrated the importance of our result for inferring the obscured AGN fraction. We found that after the anisotropy is properly accounted for, the dust covering factors show very weak dependence on L-AGN, with values in the range of approximate to 0.6-0.7. Our results also suggest a higher fraction of obscured AGNs at high luminosities than those found by X-ray surveys, in part owing to the presence of a Compton-thick AGN population predicted by population synthesis models
Dissecting the active galactic nucleus in Circinus -- II. A thin dusty disc and a polar outflow on parsec scales
Recent observations which resolved the mid-infrared (MIR) emission of nearby
active galactic nuclei (AGN), surprisingly revealed that their dust emission
appears prominently extended in the polar direction, at odds with the
expectations from the canonical dusty torus. This polar dust, tentatively
associated with dusty winds driven by radiation pressure, is found to have a
major contribution to the MIR flux from scales of a few to hundreds of parsecs.
When facing a potential change of paradigm, case studies of objects with the
best intrinsic resolution are essential. One such source with a clear detection
of polar dust is a nearby, well-known AGN in the Circinus galaxy. In the first
paper, we successfully explained the peculiar MIR morphology of Circinus
observed on large, tens of parsec scales with a model consisting of a compact
dusty disc and an extended hollow dusty cone. In this work, we further refine
the model on smaller, parsecs scales to test whether it can also explain the
MIR interferometric data. We find that a model composed of a thin dusty disc
seen almost edge-on and a polar outflow in the form of a hyperboloid shell can
reproduce well the VLTI/MIDI observations at all wavelengths, baselines and
position angles. In contrast, while providing a good fit to the integrated MIR
spectrum, the dusty torus model fails to reproduce the spatially resolved
interferometric data. We put forth the dischyperboloid wind model of
Circinus AGN as a prototype for the dust structure in the AGN population with
polar dust.Comment: MNRAS accepte
The narrow Fe K line and the molecular torus in active galactic nuclei - an IR/X-ray view
The narrow component of the iron K is an almost ubiquitous feature in
the X-ray spectra of active galactic nuclei (AGN) and is believed to originate
in neutral material, possibly located in the molecular torus. This would imply
a tight connection between the Fe K equivalent width (EW) and the
physical properties of the torus. In a recent work we have shown that the
decrease of the covering factor of the torus with the luminosity, as expected
by luminosity-dependent unification models, would be able to explain the
decrease of Fe K EW with the luminosity (i.e., the X-ray Baldwin
effect). Recent developments in the study of the mid-IR (MIR) spectrum of AGN
allow important parameters of the torus to be deduced, such as its covering
factor () and equatorial column density (),
by applying clumpy torus models. Using XMM-Newton/EPIC observations of a sample
of 24 type-I AGN, we investigate the relation between the physical parameters
of the torus obtained by recent MIR works and the properties of the Fe
K line. We correct the values of the Fe K EW by taking the
inclination angle, the photon index, the equatorial column density, and
half-opening angle of the torus into account using a physical torus model of
X-ray reprocessed radiation. We find that the relation between Fe K EW
and shows a slope that is consistent with the expected value,
albeit with a low statistical significance. A trend that is consistent with the
theoretical prediction is also found when comparing the Fe K EW to
. Our work seems to confirm that the bulk of the narrow Fe
K line is produced by the same material responsible for the MIR
emission.Comment: A&A in press, 15 pages, 5 Figures, 3 tables - Few references update
Spatial field reconstruction with INLA: Application to simulated galaxies
Aims. Monte Carlo Radiative Transfer (MCRT) simulations are a powerful tool
for understanding the role of dust in astrophysical systems and its influence
on observations. However, due to the strong coupling of the radiation field and
medium across the whole computational domain, the problem is non-local and
non-linear and such simulations are computationally expensive in case of
realistic 3D inhomogeneous dust distributions. We explore a novel technique for
post-processing MCRT output to reduce the total computational run time by
enhancing the output of computationally less expensive simulations of
lower-quality.
Methods. We combine principal component analysis (PCA) and non-negative
matrix factorization (NMF) as dimensionality reduction techniques together with
Gaussian Markov random fields and the Integrated nested Laplace approximation
(INLA), an approximate method for Bayesian inference, to detect and reconstruct
the non-random spatial structure in the images of lower signal-to-noise or with
missing data.
Results. We test our methodology using synthetic observations of a galaxy
from the SKIRT Auriga project - a suite of high resolution magneto-hydrodynamic
Milky Way-sized galaxies simulated in cosmological environment by 'zoom-in'
technique. With this approach, we are able to reproduce high photon number
reference images times faster with median residuals below .Comment: To be published in Numerical methods and codes of Astronomy and
Astrophysic