Dust in active nuclei - II. Powder or gravel?

By R. Maiolino, A. Marconi, E. Oliva. 2000

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In a companion paper, Maiolino et al. (2001) presented various observational evidences for \anomalous" dust properties in the circumnuclear region of AGNs and, in particular, the reduced EB V=NH andAV=NH ratios, the absence of the silicate absorption feature in mid-IR spectra of Sy2s and the absence of the carbon dip in UV spectra of reddened Sy1s. In this paper we discuss various explanations for these facts. The observational constraints favor a scenario where coagulation, catalyzed by the high densities in the circumnuclear region, yields to the formation of large grains. The resulting extinction curve is featureless, flatter than Galactic and the EB V=NH and AV=NH ratios are signicantly reduced. These results should warn about an inappropriate use of the standard Galactic extinction curve and AV=NH ratio when dealing with the extreme gas conditions typical of the circumnuclear clouds of AGNs. We also investigated alternative scenarios for the observed anomalous properties of dust in AGNs. Some of these scenarios might explain some of the observed properties for a few objects, but they generally fail to account for all of the observational constraints obtained for the large sample of AGNs studied in these works.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/84e4849e2a0ed69f1a6edac59c74ba2955349f92

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Citations in the corpus

Unabsorbed Seyfert 2 galaxies (2002, result): “This result confirms previous findings of Maiolino et al. (2001) who notice that a few LLAGNs in their sample were characterized by an AV /NH ratio consistent with or even higher than the Galactic value.”
The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infrared to X-Ray Spectral Energy Distribution (2003, background): “Maiolino et al. (2001a) conclude that the most likely explanation of this is that dust in the circumnuclear region of AGNs is predominantly composed of large grains that do not contribute to the feature at 9.7 m. Imanishi (2001) has argued from the probable shape of the IR L and M 0 band extinction…”
A Spitzer IRS Low-Resolution Spectroscopic Search for Buried AGNs in Nearby Ultraluminous Infrared Galaxies: A Constraint on Geometry between Energy Sources and Dust (2007, background): “In ULIRGs, it is possible that the dust extinction curve is very different from the Galactic one, because dust coagulation in the high-density environment in ULIRGs’ cores could increase typical dust grain size up to a few microns (Laor & Draine 1993; Maiolino et al. 2001a, 2001b; Imanishi 2001).”
Unusual Broad Absorption Line Quasars from the Sloan Digital Sky Survey (2002, result): “Our result is not necessarily in conflict with Maiolino et al. (2001b), who found evidence for flat extinction curves in the circumnuclear regions of Seyferts, which Maiolino, Marconi, & Oliva (2001a) interpret as being produced by dust dominated by very large grains.”
CHASING HIGHLY OBSCURED QSOs IN THE COSMOS FIELD (2008, background): “…ratios and F(24µm) as low as 40µJy, are highly ob- a From Simpson et al. (1999); b Allowing for the fact than obscured AGN and QSO can have gas-to-dust ratios much smaller than the Galactic value, see e.g. Maiolino et al. 2001 and Martinez-Sansigre et al. (2006). scured, possibly CT AGN at z=1–3.”
Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths (2015, background): “…the so-called ‘open geometry’ in CLOUDY, appropriate for gas with small covering factor (typically less than 10 per cent for AGN narrow-line regions; Maiolino et al. 2001; Baskin & Laor 2005), and describe the physical conditions of the photoionized gas in terms of the following main adjustable…”
Dust covering factor, silicate emission, and star formation in luminous QSOs (2007, background): “However, the absence of clear detections prior to theSpitzer epoch induced various authors to either postulate a very compact and dense torus (e.g. Pier & Krolik, 1993) or different dust compositions (Laor & Draine, 1993; Maiolino et al., 2001a,b).”
Spitzer Power-Law Active Galactic Nucleus Candidates in the Chandra Deep Field-North (2007, background): “…an order of magnitude lower than that of the Milky Way (e.g. Maccacaro, Perola, & Elvis 1982; Reichert et al. 1985; Granato et al. 1997; Maiolino et al. 2001a), due either to a higher gas-to-dust ratio, or to the formation of large grains in the dense AGN environment (Maiolino et al. 2001a, 2001b).”
ISM properties in low-metallicity environments. III. The dust spectral energy distributions of II Zw 40, He 2-10 and NGC 1140 (2005, result): “These differerent results obtained for global galactic scales versus more local regions around the SSCs is likely due to the destruction of small grains around SSCs, as is also observed in the vicinity of AGNs (Maiolino et al. 2001a,b).”
The innermost region of AGN tori: implications from the HST/NICMOS type 1 point sources and near-IR reverberation (2007, background): “Note that a dominance of large grains in AGN tori has been suggested and discussed on different grounds (e.g. Maiolino et al. 2001a,b; Gaskell et al. 2004).”
Three-dimensional radiative transfer models of clumpy tori in Seyfert galaxies (2008, background): “Although several hints (Maiolino et al. 2001a,b; Jaffe et al. 2004) point to the possibility that dust in the nuclear regions f AGN is dominated by large grains, we will limit our present investigation to the classic MRN-model (Mathis et al. 1977) for three reasons: first, and most important, we…”
X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies (2005, background): “…to variability in the absorption column density, and/or geometry, and/or ionization state (Malizia et al. 1997, Piro 1999, Risaliti, Elvis & Nicastro 2002, Matt, Guainazzi & Maiolino 2003), or unusual dust-to-gas ratios have been proposed to explain such differences (Maiolino et al. 2001a, 2001b).”