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 of ∼15,000 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 and MIR photometry from the preliminary data release of the Wide-field Infrared Survey Explorer. 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 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. © 2011. The American Astronomical Society. All rights reserved.