Hot Graphite Dust and the Infrared Spectral Energy Distribution of
  Active Galactic Nuclei
  We present a detailed investigation of the near-to-far infrared (IR) spectral
energy distribution (SED) of a large sample of Spitzer-observed active galactic
nuclei (AGN). We fitted the spectra of 51 narrow line Seyfert 1 galaxies
(NLS1s) and 64 broad line Seyfert 1 galaxies (BLS1s) using a three component
model: a clumpy torus, a dusty narrow line region (NLR) and hot pure-graphite
dust located in the outer part of the broad line region (BLR). The fitting is
performed on star formation (SF) subtracted SEDs using SF templates that take
into account the entire range of possible host galaxy properties. We find that
the mid-IR intrinsic emission of NLS1s and BLS1s are very similar, regardless
of the AGN luminosity, with long wavelength downturn at around 20-25 um. We
present a detailed model of the hot dust component that takes into account the
distribution of dust temperature within the clouds and their emission line
spectrum. The hot dust continuum provides a very good fit to the observed
near-IR continuum spectrum. Most line emission in this component is
dramatically suppressed, except MgII(2798A) and HeI lines that are still
contributing significantly to the total BLR spectrum. We calculate the covering
factors of all the AGN components and show that the covering factor of the
hot-dust clouds is about 0.15-0.35, similar to the covering factor of the
torus, and is anti-correlated with the source luminosity and the normalized
accretion rate.
