Infrared spectra of obscuring dust tori around active galactic nuclei. I - Calculational method and basic trends
TL;DR: In this article, the authors used a 2D radiative transfer algorithm to calculate the thermally reradiated infrared spectra of the compact dust tori which are thought to surround many AGN.
Abstract: Using a new 2D radiative transfer algorithm, we have calculated the thermally reradiated infrared spectra of the compact dust tori which are thought to surround many AGN. These tori radiate anisotropically. Face-on tori may be from one-half to a few orders of magnitude brighter than edge-on tori throughout the infrared. Their spectra at nearly all viewing angles are basically 'bumps' which are about 50 percent wider than blackbodies and peak in the mid-infrared at a wavelength determined mainly by the flux of nuclear radiation on the inner edge of the torus. The infrared color temperatures are hotter for face-on tori than edge-on tori by about 100 K. The 10 micron silicate feature often associated with dust can appear in absorption, emission, or not at all. There is a rough tendency for absorption features to be more prominent for edge-on tori than for face-on tori.
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20 Apr 2005
TL;DR: Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium New observations are revealing the ubiquity of this process, particularly at high redshift as discussed by the authors.
Abstract: Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium New observations are revealing the ubiquity of this process, particularly at high redshift We describe the physics behind these winds, discuss the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe, and consider the implications of energetic winds for the formation and evolution of galaxies and the intergalactic medium To inspire future research, we conclude with a set of observational and theoretical challenges
1,453 citations
TL;DR: In this article, a clumpy torus model with N{sub 0} {approx} 5-15 dusty clouds along radial equatorial rays successfully explain AGN infrared observations.
Abstract: Clumpy torus models with N{sub 0} {approx} 5-15 dusty clouds along radial equatorial rays successfully explain AGN infrared observations. The dust has standard Galactic composition, with individual cloud optical depth {tau}{sub V} {approx} 30-100 at visual. The models naturally explain the observed behavior of the 10 {mu}m silicate feature, in particular the lack of deep absorption features in AGNs of any type, and can reproduce the weak emission feature tentatively detected in type 2 QSOs. The clouds' angular distribution must have a soft edge, e.g., Gaussian, and the radial distribution should decrease as 1/r or 1/r{sup 2}. In line with recent interferometry, the ratio of the torus outer to inner radius can be as small as {approx}5-10. The models can produce nearly isotropic IR emission together with highly anisotropic obscuration, as required by observations. Clumpiness implies that the viewing angle determines an AGN classification only probabilistically; 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 N{sub 0}, and this fraction's observed decrease with luminosity can be explained with a decrease of either parameter. X-ray obscuration,more » too, is probabilistic; resulting from both dusty and dust-free clouds, it might be dominated by the latter, giving rise to the observed QSOs that are X-ray obscured. Observations indicate that the torus and broad-line-emitting clouds form a seamless distribution, with the transition between the two caused by dust sublimation. Torus clouds may have been detected in the outflow component of H{sub 2}O maser emission from two AGNs. Proper-motion measurements of outflow masers, especially in Circinus, are a promising method for probing the morphology and kinematics of torus clouds.« less
822 citations
TL;DR: In this paper, it was shown that in a clumpy medium, a large range of dust temperatures coexist at the same distance from the radiation central source, which explains the low dust temperatures found close to the nucleus of NGC 1068 in 10� m interferometric observations.
Abstract: Accordingtounifiedschemesof activegalactic nuclei(AGNs), thecentralengine issurroundedbydusty,optically thickcloudsinatoroidalstructure.Wehaverecentlydevelopedaformalismthatforthefirsttimetakesproperaccount of the clumpy nature of the AGN torus. We now provide a detailed report of our findings in a two-paper series. Here wepresentourgeneralformalismforradiativetransferinclumpymediaandconstructitsbuildingblocksfortheAGN problem—the source functions of individual dusty clouds heated by the AGN radiation field. We show that a fundamental difference from smooth density distributions is that in a clumpy medium, a large range of dust temperatures coexist at the same distance from the radiation central source. This distinct property explains the low dust temperatures found close to the nucleus of NGC 1068 in 10� m interferometric observations. Wefind that, irrespective of the overallgeometry,aclumpydustdistributionshowsonlymoderate variationinitsspectralenergydistribution, andthe 10� mabsorptionfeatureisneverdeep.Furthermore,theX-rayattenuatingcolumndensityiswidelyscatteredaround the column density that characterizes the IR emission. All of these properties are characteristic of AGN observations. The assembly of clouds into AGN tori and comparison with observations are presented in the companion paper. Subject headingg dust, extinction — galaxies: active — galaxies: Seyfert — infrared: general — quasars: general — radiative transfer
627 citations
Cites methods or result from "Infrared spectra of obscuring dust ..."
...However, because of the difficulties in handling clumpy media, models of the torus IR emission, beginning with Pier & Krolik (1992), utilized smooth density distributions instead....
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...This behavior conflicts with the results of smooth-density models (e.g., Pier & Krolik 1992) but is a natural consequence of clumpiness: as is evident from figures 11 and 12, single clumps never produce extremely deep features....
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TL;DR: A review of recent developments related to the unified model of active galactic nuclei (AGNs) can be found in this paper, where the authors focus on new ideas about the origin and properties of the central obscurer (torus) and the connection to its surroundings.
Abstract: This review describes recent developments related to the unified model of active galactic nuclei (AGNs). It focuses on new ideas about the origin and properties of the central obscurer (torus) and the connection to its surroundings. The review does not address radio unification. AGN tori must be clumpy but uncertainties about their properties persist. Today's most promising models involve disk winds of various types and hydrodynamic simulations that link the large-scale galactic disk to the inner accretion flow. Infrared (IR) studies greatly improved our understanding of the spectral energy distribution of AGNs, but they are hindered by various selection effects. X-ray samples are more complete. The dependence of the covering factor of the torus on luminosity is a basic relationship that remains unexplained. There is also much confusion regarding real type-II AGNs, which do not fit into a simple unification scheme. The most impressive recent results are due to IR interferometry, which is not in accord wit...
610 citations
Cites background from "Infrared spectra of obscuring dust ..."
...There are three generic types of phenomenological torus models: continuous (or smooth) gas distribution models, (e.g. Pier & Krolik 1992; Granato, Danese & Franceschini 1997; Efstathiou & Rowan-Robinson 1995; Dullemond & van Bemmel 2005; Fritz, Franceschini & Hatziminaoglou 2006), clumpy torus…...
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...…about the size, composition and geometry of the torus in an attempt to explain the observations, mostly the spatially resolved images and the NIR-MIR SED (e.g. Pier & Krolik 1992; Fritz, Franceschini & Hatziminaoglou 2006; Nenkova et al. 2008a; Hönig et al. 2010; Stalevski et al. 2012)....
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TL;DR: Interferometric mid-infrared observations that spatially resolve dust structures in the galaxy NGC 1068 reveal warm dust in a structure 2.1 parsec thick and 3.4 parsec in diameter, surrounding a smaller hot structure.
Abstract: Active galactic nuclei (AGNs) display many energetic phenomena—broad emission lines, X-rays, relativistic jets, radio lobes—originating from matter falling onto a supermassive black hole. It is widely accepted that orientation effects play a major role in explaining the observational appearance of AGNs. Seen from certain directions, circum-nuclear dust clouds would block our view of the central powerhouse1,2. Indirect evidence suggests that the dust clouds form a parsec-sized torus-shaped distribution. This explanation, however, remains unproved, as even the largest telescopes have not been able to resolve the dust structures. Here we report interferometric mid-infrared observations that spatially resolve these structures in the galaxy NGC 1068. The observations reveal warm (320 K) dust in a structure 2.1 parsec thick and 3.4 parsec in diameter, surrounding a smaller hot structure. As such a configuration of dust clouds would collapse in a time much shorter than the active phase of the AGN3, this observation requires a continual input of kinetic energy to the cloud system from a source coexistent with the AGN.
595 citations