Dusty Disks and the Infrared Emission from AGN

Abstract

The distortions inferred in the gaseous disks of active galaxies suggest that a significant, and possibly dominant fraction of the 1 - 1000μm radiation observed from AGN must be thermal emission from gas and dust heated by the central source. We report calculations of the growth and sublimation of dust grains in the outer parts of accretion disks appropriate to AGN. The thermal state of the gas undergoes a sudden change at the radius where the dust sublimates. The outer portion of the accretion disk radiates at 0.5 – 5 μm; free-free emission from gas whose dust has sublimated contributes to the flux at 0.5 – 2μm. If this thermal emission dominates the flux from radio-quiet quasars, it naturally explains the frequency and depth of the universal minimum in νF_ν at 10^(14.5) Hz. Free-free emission from the photoionized surface layers of the disk at larger radii produces a radio flux at ν ≲ 10^(11) HZ comparable to that observed in radio-quiet quasars. The far-infrared and submillimeter emission from radio-quiet quasars and Seyfert galaxies is more naturally interpreted as reradiation by dust, than as nonthermal emission from the inner accretion disk.