Hot Dust Clouds with Pure Graphite Composition around Type-I Active
  Galactic Nuclei
  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.
