Hot-Dust Clouds With Pure-Graphite Composition Around Type-I Active Galactic Nuclei

Abstract

We fitted the optical to mid-infrared (MIR) spectral energy distributions (SEDs) of ∼15000 type-I, 0.75 < z < 2, active galactic nuclei (AGNs) in an attempt to constrain the properties of the physical component responsible for the rest-frame near-infrared (NIR) emission. We combine optical spectra from the Sloan Digital Sky Survey (SDSS) and MIR photometry from the preliminary data release of the Wide Infrared Survey Explorer (WISE). The sample spans a large range of AGN properties: luminosity, black hole mass, and accretion rate. Our model has two components: a UV-optical continuum source and very hot, pure-graphite dust clouds. We present the luminosity of the hot-dust component and its covering factor, for all sources, and compare it with the intrinsic AGN properties. We find that the hot-dust component is essential to explain the (rest) NIR emission in almost all AGNs in our sample, and that it is consistent with clouds containing pure-graphite grains and located between the dust-free broad line region (BLR) and the "standard" torus. The covering factor of this component has a relatively narrow distribution around a peak value of ∼0.13, and it correlates with the AGN bolometric luminosity. We suggest that there is no significant correlation with either black hole mass or normalized accretion rate. The fraction of hot-dust-poor AGNs in our sample is ∼ 15 − 20%, consistent with previous studies. We do not find a dependence of this fraction on redshift or source luminosity.

Reaching the peak of the quasar spectral energy distribution – II. Exploring the accretion disc, dusty torus and host galaxy

“…This is slightly higher than the Landt et al (2011) value, but is consistent with our result, which lies between the Landt et al (2011) and Mor & Trakhtenbrot (2011) values. The Mor & Trakhtenbrot (2011) sample covers a larger range of luminosities than ours but they do not find a dependence of covering factor on M BH orṁ.…”

“…Mor & Trakhtenbrot (2011) found their distribution of hot dust covering factor values peaked at ∼13 per cent, in a sample of 15 000 SDSS AGN, fitting WISE photometry of comparable quality to our sample. This is slightly higher than the Landt et al (2011) value, but is consistent with our result, which lies between the Landt et al (2011) and Mor & Trakhtenbrot (2011) values. The Mor & Trakhtenbrot (2011) sample covers a larger range of luminosities than ours but they do not find a dependence of covering factor on M BH orṁ.…”

“…Finally, Roseboom et al (2013) inferred a broad distribution of covering factors generally greater than those measured in this work, Landt et al (2011) and Mor & Trakhtenbrot (2011). They fit the AGN component using Elvis et al (1994) SED templates, rather than the physically motivated model we employ for our analysis, and this may lead them to underestimate the AGN luminosity and predict higher covering factors.…”

Composite Spectral Energy Distributions and Infrared–Optical Colors of Type 1 and Type 2 Quasars

“…The completion of the Wide-field Infrared Survey Explorer (WISE; Wright et al 2010) survey of the full sky has allowed us to make dramatic progress in statistical studies of AGNs (e.g., Mor & Trakhtenbrot 2011;Donoso et al 2012;Edelson & Malkan 2012;Eisenhardt et al 2012;Ichikawa et al 2012;Mateos et al 2012;Plotkin et al 2012;Sajina et al 2012;Stern et al 2012;Assef et al 2013;Geach et al 2013;DiPompeo et al 2014DiPompeo et al , 2015DiPompeo et al , 2016DiPompeo et al , 2017Assef et al 2015Assef et al , 2017Secrest et al 2015). In particular, WISE enables us to detect and characterize hundreds of thousands of obscured quasars, increasing the sizes of obscured quasar samples by two orders of magnitude and allowing the first large statistical studies of their properties.…”

The Covering Factor of Warm Dust in Weak Emission-Line Active Galactic Nuclei

“…Niko lajuk & Walter (2012) estimated the covering factor of the BLR in WLAGNs from the equivalent width (EW) of C IV emission line and the ratio of X-ray to optical luminosity. They found that the covering factors of BLRs in WLAGNs are lower than those in normal quasars by a factor of 10.…”