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

Abstract

We describe observations of 9.7 m 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).

X-ray observations of highly obscuredτ9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?

“…This result can be seen more clearly in Fig. 4, where we plot the 9.7 μm optical depth as a function of the X-ray column density for both the 12 μm sample as well as the sample of Shi et al (2006). We note that the latter is based mainly on the optically selected sample of Risaliti et al (1999).…”

“…We discuss this in detail in the following section. Shi et al (2006) described observations of 9.7 μm features in 97 AGN including a wide range of types. They found that the strength of the silicate feature correlates with the hydrogen column density, derived from X-ray observations in the sense that the low column densities correspond to silicate emission, while high columns correspond to silicate absorption.…”

“…However, the mid-IR spectrum shows no prominent 9.7 μm absorption (Sturm et al 2000). In Shi et al (2006) there are many more examples of Compton-thick nuclei which are optically thin at 9.7 μm (see Fig. 4).…”

“…Nenkova et al 2008;Wada et al 2009;Nikutta et al 2009), the X-ray emission from the AGN could be obscured by a circumnuclear cloud, while the infrared emission, which is produced at larger distances from the AGN, could experience low or no obscuration at all (see Fig. 5 in Shi et al 2006). Spoon et al (2007) proposed that the depth of the Si 9.7 μm feature can be used as a tool to predict the obscuring dust distribution.…”

Dust tori in radio galaxies

“…If this unification scenario is correct, infrared observations of the nuclei of radio galaxies may act as calorimeters that indicate the levels of accretion activity. The dust tori, believed to be clumpy and to extend out to radii of tens to hundreds of parsecs (Krolik & Begelman 1988;Nenkova et al 2002;Granato et al 1997;Shi et al 2006), reradiate the optical and ultraviolet emission originating from the accretion disks surrounding the supermassive black holes (>10 6 M ). However, only radio galaxies displaying high-excitation narrow-line optical emission, referred to as high-excitation radio galaxies (HEGs), are understood to host obscured broad line and bright continuum emission, and to be in the process of active black hole accretion (Barthel 1994;Laing et al 1994).…”