AGN Dusty Tori. I. Handling of Clumpy Media

By M. Nenkova, M. Sirocky, Željko Ivezic´, M. Elitzur. 2008

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According to unified schemes of active galactic nuclei (AGNs), the central engine is surrounded by dusty, optically thick clouds in a toroidal structure. We have recently developed a formalism that for the first time takes proper account of the clumpy nature of the AGN torus. We now provide a detailed report of our findings in a two-paper series. Here we present our general formalism for radiative transfer in clumpy media and construct its building blocks for the AGN 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 μ m interferometric observations. We find that, irrespective of the overall geometry, a clumpy dust distribution shows only moderate variation in its spectral energy distribution, and the 10 μ m absorption feature is never deep. Furthermore, the X-ray attenuating column density is widely scattered around 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/4e9ae52b2285fb92dfe2d4a5000d63fd49c89a34

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Citations in the corpus

Dusty Star Forming Galaxies at High Redshift (2014, background): “…Efstathiou and Rowan-Robinson, 2003; Efstathiou and Siebenmorgen, 2009; Nenkova et al., 2002; Dullemond and van Bemmel, 2005; Piovan et al., 2006; Nenkova et al., 2008; Takagi et al., 2003; Fritz et al., 2006; Hönig et al., 2006; Schartmann et al., 2008), but here we try to focus on the…”
A NEW NETWORK FOR HIGHER-TEMPERATURE GAS-PHASE CHEMISTRY. I. A PRELIMINARY STUDY OF ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI (2010, methodology): “The luminosity for NGC 1068 was actually obtained by fitting a clumpy model to observational data on the dusty torus in NGC 1068 (Mason et al. 2006; Nenkova et al. 2008), which shows the dust temperature to be 800 K in the inner sub-pc torus and 300 K for the few-pc-sized torus (Jaffe et al. 2004;…”
The dusty heart of nearby active galaxies. II. From clumpy torus models to physical properties of dust around active galactic nuclei (2009, methodology): “Nenkova et al. (2008b) presented model SEDs simulated via this approach which are in general agreement with observations.”
Revisiting the Unified Model of Active Galactic Nuclei (2015, methodology): “This approach was later used by Mor & Netzer (2012) who combined the Nenkova et al. (2008b) clumpy torus model with two additional components, one containing pure graphite grains and the other a dusty NLR.”
MID-INFRARED SELECTION OF ACTIVE GALACTIC NUCLEI WITH THE WIDE-FIELD INFRARED SURVEY EXPLORER. II. PROPERTIES OF WISE-SELECTED ACTIVE GALACTIC NUCLEI IN THE NDWFS BOÖTES FIELD (2012, background): “For example, several authors (e.g., Krolik & Begelman 1988; Nenkova et al. 2002, 2008; Elitzur & Shlosman 2006; Tristram et al. 2007) have argued that the dust in the torus must be in optically and geometrically thick clumps to reproduce observations, while others (e.g., Dullemond & van Bemmel…”
Stellar Population Inference with Prospector (2020, methodology): “Circumstellar dust around AGB stars is implemented via the Villaume et al. (2015) templates, while dust associated with AGN is modeled using a simplified implementation of the Nenkova et al. (2008b) templates.”
First X-ray-based statistical tests for clumpy-torus models: eclipse events from 230 years of monitoring of Seyfert AGN (2014, methodology): “For instance, if N0 is between 5 and 15 (see, e.g., Nenkova et al. 2008b), it is clear from Fig.”
A systematic search for changing-look quasars in SDSS (2015, background): “Another possibility is that the entire UV/continuum emitting region and BLR is obscured; this might be expected if the distribution of dust lying at the outskirts of the BLR is patchy (e.g., Nenkova et al. 2008a,b; Elitzur 2012).”
Resolving the mid-infrared cores of local Seyferts (2009, background): “This suggested, instead, that dust clouds have significant clumpiness in the AGN tori (e.g., Dullemond & van Bemmel 2005; Hönig et al. 2006; Nenkova et al. 2008a,b; Schartmann et al. 2008) which leads to the observed mid-IR emission from hot illuminated cloud faces in the inner torus regions being…”
Erratum: Updating quasar bolometric luminosity corrections (2012, background): “Nenkova et al. (2008) compare dusty torus models (Nenkova et al. 2008) to observations of type 1 and type 2 sources and find general agreement for opening angles in the range 40◦ − 60◦.”
Molecular line emission in NGC 1068 imaged with ALMA: I. An AGN-driven outflow in the dense molecular gas (2014, methodology): “Alonso-Herrero et al. (2011) fitted the nuclear NIR and MIR SED and MIR spectroscopy of NGC 1068 using the so-called CLUMPY torus models of Nenkova et al. (2008a, 2008b).”
The subarcsecond mid-infrared view of local active galactic nuclei – II. The mid-infrared–X-ray correlation (2015, background): “Current clumpy and smooth torus models (e.g., Nenkova et al. 2008; Hönig & Kishimoto 2010; Stalevski et al. 2012) predict differences of ∼ 0.3 dex in the MIR–X-ray ratio with changing inclination angles assuming isotropic X-ray/UV emission.”
IDENTIFYING LUMINOUS ACTIVE GALACTIC NUCLEI IN DEEP SURVEYS: REVISED IRAC SELECTION CRITERIA (2012, background): “While the observed degree of MIR anisotropy therefore remains an open question, a low optical depth is a natural consequence of clumpy torus models, which predict only weak NIR-MIR anisotropies that essentially disappear by λ∼ 12µm (e.g. Nenkova et al. 2008a,b).”
Canonical high power blazars (2009, background): “We emphasise that our treatment of the torus emission is approximate: it is likely that the torus itself is a complex structure, possibly clumpy (Nenkova et al. 2008) with a range of radii, extending also quite close to the black hole, where the temperatur is just below dust sublimation (i.e.∼ 1500…”
Discovery of 21 New Changing-look AGNs in the Northern Sky (2017, background): “…] 4 J un 2 01 8 2 movement of obscuring material, in a scenario that the dusty toroidal structure obscuring the BLR has a patchy distribution (e.g., Nenkova et al. 2008a,b; Elitzur 2012) or an accelerating outflow (e.g., Shapovalova et al. 2010); (2) variable accretion rate, in an evolutionary…”
Hot graphite dust and the infrared spectral energy distribution of active galactic nuclei (2011, methodology): “We use the clumpy torus models of Nenkova et al. (2008a) in a similar way to that described in M09.”
Constraining the properties of AGN host galaxies with spectral energy distribution modelling (2015, background): “The second one is that it has been shown in Feltre et al. (2012) that the smooth torus library of Fritz et al. (2006) is highly degenerated with the clumpy torus library of Nenkova et al. (2008).”
TESTING THE UNIFICATION MODEL FOR ACTIVE GALACTIC NUCLEI IN THE INFRARED: ARE THE OBSCURING TORI OF TYPE 1 AND 2 SEYFERTS DIFFERENT? (2011, methodology): “The latter can be represented in terms of the escape probability Pesc (see equation 4 in Nenkova et al. 2008a).”
ON THE UNIFICATION OF ACTIVE GALACTIC NUCLEI (2012, background): “…concept of “torus covering factor”, CT, can be rigorously defined as the fraction of the sky at the AGN center covered by obscuring material; it is the same as the fraction of randomly distributed observers whose view to the center is blocked, and thus see a type 2 AGN (Nenkova et al. 2008a).”