Dust in active nuclei. I. evidence for anomalous properties

By R. Maiolino, A. Marconi, M. Salvati, G. Risaliti, P. Severgnini, E. Oliva, F. Franca, L. Vanzi. 2000

331 citations recorded by the source corpus.

We present observational evidences that dust in the circumnuclear region of AGNs has different properties than in the Galactic diffuse interstellar medium. By comparing the reddening of optical and infrared broad lines and the X-ray absorbing column density we find that the E(B-V)/N_H ratio is nearly always lower than Galactic by a factor ranging from ~3 up to ~100. Other observational results indicate that the Av/N_H ratio is significantly lower than Galactic in various classes of AGNs including intermediate type 1.8-1.9 Seyferts, hard X-ray selected and radio selected quasars, broad absorption line QSOs and grism selected QSOs. The lack of prominent absorption features at 9.7um (silicates) and at 2175A (carbon dip) in the spectra of Seyfert 2s and of reddened Seyfert 1s, respectively, add further evidence for dust in the circumnuclear region of AGNs being different from Galactic. These observational results indicate that the dust composition in the circumnuclear region of AGNs could be dominated by large grains, which make the extinction curve flatter, featureless and are responsible for the reduction of the E(B-V)/N_H and Av/N_H ratios. Regardless of the physical origin of these phenomena, the reduced dust absorption with respect to what expected from the gaseous column density should warn about a mismatch between the optical and the X-ray classification of the active galactic nuclei in terms of their obscuration.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/52681edfe85902c6acaf48a1a6133bcff6f6e052

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1. D. M. Alexander
2. H. Netzer
3. P. Hopkins
4. L. Hernquist
5. R. Hickox
6. Luis C. Ho
7. W. N. Brandt
8. R. Maiolino

Citations in the corpus

• X-ray obscuration and obscured AGN in the local universe (2005, background): “This corresponds to NH ≃0.5– 1.0×1023 cm−2 for gas-to-dust ratios typically observed in luminous and obscured nearby AGN (Granato et al. 1997, Maiolino et al. 2001)5.”
• The HELLAS2XMM Survey. II. Multiwavelength Observations of P3: An X-Ray-bright, Optically Inactive Galaxy (2002, background): “…present findings indicate that a mildly obscured AGN could be hosted by P3, we note that hard X–ray selected objects withLX > 1042 erg s−1 are generally characterized (with some exceptions, e.g. Brandt et al. 1997) by a dust to gas ratio much lower than the Galactic standard (Maiolino et al. 2001).”
• ISM properties in low-metallicity environments II. The dust spectral energy distribution of NGC 1569 (2003, background): “The presence of smaller grain size distributions, that dominate the global IR emission in dwarf galaxies, is opposite to the deduced dust properties in AGNs (e.g. Laor & Draine 1993; Maiolino et al. 2001).”
• AGN Dusty Tori. II. Observational Implications of Clumpiness (2008, background): “Furthermore, Maiolino et al. (2001) find that the X-ray-absorbing column exceeds the reddening column in each member of an AGN sample by a factor ranging from 3 up to 100, implying that the bulk of the X-ray absorption comes from the clouds in the dust-free inner portion of the torus.”
• Resolving the mid-infrared cores of local Seyferts (2009, background): “G A We present new photometry of 16 local Seyferts including 6 Compt n-thick sources inN-band filters around 12-µm, obtained with the VISIR instrument on the 8-m Very Large Telescope.”
• Evidence for Quasar Activity Triggered by Galaxy Mergers in HST Observations of Dust-reddened Quasars (2007, background): “While dusty, it is well within the 3Y100 times Galactic value range deduced by Maiolino et al. (2001).”
• First X-ray-based statistical tests for clumpy-torus models: eclipse events from 230 years of monitoring of Seyfert AGN (2014, background): “…X-ray-absorbed type Is, Perola et al. 2004; Garcet et al. 2007); a complication is that optical obscuration probes only dusty gas, while X-ray absorption probes both dusty and dust-free gas, and X-ray columns can exceed inferred optical-reddening columns by factors of 3–100 (Maiolino et al. 2001).”
• 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).”
• Faint high-redshift AGN in the Chandra deep field south: the evolution of the AGN luminosity function and black hole demography (2011, methodology): “This is also lower than in the (mostly low-z) AGN studied by Maiolino et al. (2001) and Shi et al. (2006).”
• 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).”
• 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).”
• LINE DERIVED INFRARED EXTINCTION TOWARD THE GALACTIC CENTER (2011, background): “However, there are AGNs with different X-ray absorption to AV ratios (Maiolino et al. 2001; Li 2007).”
• The bolometric quasar luminosity function at z = 0–7 (2020, background): “In addition, some observations have also shown that the extinction curve of AGN may be much shallower than the commonly assumed SMC-like extinction curve (e.g., Maiolino et al. 2001; Czerny et al. 2004).”
• 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).”
• Towards a physical model of dust tori in Active Galactic Nuclei (2005, background): “Therefore, some authors (e. g. Laor & Draine 1993; Maiolino et al. 2001) claim that within an AGN dust model, small grains should be depleted.”
• The Rise of Faint, Red Active Galactic Nuclei at z > 4: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields (2025, neutral): “Using the median of the ratios reported in R. Maiolino et al. ( 2001 ) .5 , which agrees with our measured value, within the errors, better than the 21.6 cm − 2 obtained using the Galactic standard ratio.”
• Ubiquitous Variability of X-Ray-absorbing Column Densities in Seyfert 2 Galaxies (2001, background): “This possibility is interesting, given the growing evidence suggesting a low dust-to-gas ratio in the absorbing medium of many AGNs (Maiolino et al. 2001; Risaliti et al. 2001).”
• Revisiting the Unified Model of Active Galactic Nuclei (2015, background): “For example, the observed optical reddening is much smaller than the value obtained by translating the measured X-ray column to foreground extinction assuming galactic dust-to-gas ratio (e.g. Maiolino et al. 2001; Risaliti, Elvis & Nicastro 2002; Goulding et al. 2012).”