Hot graphite dust and the infrared spectral energy distribution of active galactic nuclei

We present a detailed investigation of the near-infrared to far-infrared (near-IR to far-IR) spectral energy distribution (SED) of a large sample of Spitzer-observed active galactic nuclei (AGNs). 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 and hot pure-graphite dust located in the outer part of the broad-line region (BLR). The fitting was 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 is very similar, regardless of the AGN luminosity, with long-wavelength downturn at around 20-25 mu m. 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 lambda 2798 and He I lines that are still contributing significantly to the total BLR spectrum. We calculate the covering factors (CFs) of all the AGN components and show that the CF of the hot-dust clouds is about 0.15-0.35, similar to the CF of the torus, and is anticorrelated with the source luminosity and the normalized accretion rate.