9.7 μm Silicate Features in Active Galactic Nuclei: New Insights into Unification Models

By Yong Shi, G. Rieke, D. Hines, V. Gorjian, M. Werner, K. Cleary, F. Low, Paul S. Smith, J. Bouwman. 2006

135 citations recorded by the source corpus.

We describe observations of 9.7 μm silicate features in 97 AGNs, exhibiting a wide range of AGN types and of X-ray extinction toward the central nuclei. We find that the strength of the silicate feature correlates with the H I column density estimated from fitting the X-ray data, such that low H I columns correspond to silicate emission, while high columns correspond to silicate absorption. The behavior is generally consistent with unification models in which the large diversity in AGN properties is caused by viewing-angle-dependent obscuration of the nucleus. Radio-loud AGNs and radio-quiet quasars follow roughly the correlation between H I columns and the strength of the silicate feature defined by Seyfert galaxies. The agreement among AGN types suggests a high-level unification with similar characteristics for the structure of the obscuring material. We demonstrate the implications for unification models qualitatively with a conceptual disk model. The model includes an inner accretion disk (<0.1 pc in radius), a middle disk (0.1-10 pc in radius) with a dense diffuse component and with embedded denser clouds, and an outer clumpy disk (10-300 pc in radius).

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b9b1a8ad502a1fff683d12d76f58f041f893c009

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In Astronomy and Astrophysics

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

In Instrumentation

1. D. M. Alexander
2. H. Netzer
3. R. Hickox
4. A. Koekemoer
5. R. Maiolino
6. P. Hopkins
7. Mara Salvato
8. J. Fritz

Citations in the corpus

• MID-INFRARED SPECTROSCOPY OF SUBMILLIMETER GALAXIES: EXTENDED STAR FORMATION IN MASSIVE HIGH-REDSHIFT GALAXIES (2009, unknown): “7µm >∼ 1.5; Shi et al. 2006; Hao et al. 2007).”
• Resolving the mid-infrared cores of local Seyferts (2009, background): “7 ∼ 1 in their small sample (similar spreads have been inferred by Shi et al. 2006 and Hao et al. 2007).”
• Spitzer Power-Law Active Galactic Nucleus Candidates in the Chandra Deep Field-North (2007, background): “Shi et al. (2006) also find several Compton-thick AGN that do not follow an observed correlation between X-ray column density and the strength of the silicate emission or absorption feature.”
• Mid-Infrared Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width (2006, methodology): “This sample is compared to samples of AGN and starburst templates from the IRS GTO programs 14 (Brandl et al. 2006; Weedman et al. 2005) and 96 and to selected AGNs from the GTO programs 82 and 86 (Shi et al. 2006).”
• The Distribution of Silicate Strength in Spitzer Spectra of AGNs and ULIRGs (2006, methodology): “The AGN sample combines the unpublished Guaranteed Time Observation (GTO) archival data (program 14) and published data from Hao et al. (2005), Weedman et al. (2005), Buchanan et al. (2006), and Shi et al. (2006).”
• 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 100 pc ELLIPTICAL AND TWISTED RING OF COLD AND DENSE MOLECULAR CLOUDS REVEALED BY HERSCHEL AROUND THE GALACTIC CENTER (2011, background): “…few parsecs, (e.g. Jaffe et al. 100000 200000 300000 2004, Radomski et al. 2008), but there is additional evidence from Spitzer imaging and spectroscopy suggesting that larger and more persistent structures play a role in shaping observed AGN properties (e.g. Shi et al. 2006, Quillen et al. 2006).”
• Dust covering factor, silicate emission, and star formation in luminous QSOs (2007, methodology): “Shi et al. (2006) provide a measure of the silicate feature strength (whose definition was adopted also by us).”
• The dusty heart of nearby active galaxies. II. From clumpy torus models to physical properties of dust around active galactic nuclei (2009, background): “The 10.0µm central wavelength seems to be more consistent with peak silicate emission features of type 1 AGN as observed withSpitzer, while absorption features in type 2s usually have their center at 9.7µm (e.g. Shi et al. 2006).”
• Three-dimensional radiative transfer modeling of AGN dusty tori as a clumpy two-phase medium (2011, background): “The toroidal geometry also explains several other observablessuch as, the presence of ionizing cones (Pogge 1988, 1989; Tadhunter & Tsvetanov 1989) and high hydrogen column densities in the X-ray, usually associated with type 2 sources (e.g.Shi et al. 2006).”
• Three-dimensional radiative transfer models of clumpy tori in Seyfert galaxies (2008, background): “Combining X-ray absorbing column densities with spectral information further strengthens the claim for a clumpy distribution of the dust (Shi et al. 2006).”
• Smooth and clumpy dust distributions in AGN: a direct comparison of two commonly explored infrared emission models (2012, background): “The controversy on the prominence of the feature was partly solved by observations carried out with Spitzer/IRS (e.g. Siebenmorgen et al. 2005; Hao et al. 2005; Shi et al. 2006).”
• Active galactic nuclei: what’s in a name? (2017, background): “The unambiguous observations of the silicate feature at 9.7 µm in emission in many known AGN (Hao et al. 2005; Siebenmorgen et al. 2005; Sturm et al. 2005; Buchanan et al. 2006; Shi et al. 2006) came as the long sought confirmation of the unified scheme.”
• TORUS AND ACTIVE GALACTIC NUCLEUS PROPERTIES OF NEARBY SEYFERT GALAXIES: RESULTS FROM FITTING INFRARED SPECTRAL ENERGY DISTRIBUTIONS AND SPECTROSCOPY (2011, background): “Shi et al. (2006) found that the 9.7µm silicate features measured from Spitzer/IRS spectroscopy of a large sample of AGN vary from emission to absorption with increasing neutral hydrogen column densities.”