Obscuration of Active Galactic Nuclei by Circumnuclear Starbursts

Abstract

We examine the possibility of active galactic nucleus (AGN) obscuration by dusty gas clouds that spurt out from circumnuclear starburst regions. For this purpose, the dynamical evolution of gas clouds is pursued, including the effects of radiation forces by an AGN as well as a starburst. Here we solve the radiative transfer equations for clouds, taking into consideration the growth of clouds by inelastic cloud-cloud collisions and the resultant change in optical depth. As a result, if the starburst is more luminous than the AGN, gas clouds are distributed extensively above the galactic disk with the assistance of radiation pressure from the starburst. The total covering factor of gas clouds reaches a maximum of about 20%. After several 107 yr, gas clouds with larger optical depth form by cloud-cloud collisions; thereafter the clouds fall back as a result of weakened radiation pressure. The larger clouds undergo runaway growth and are eventually distributed around the equatorial plane on the inner sides of circumnuclear starburst regions. These clouds have an optical depth of several tens. The result is qualitatively consistent with the putative tendency that Seyfert 2 galaxies appear more frequently associated with starbursts than Seyfert 1 galaxies. On the other hand, if the AGN luminosity overwhelms that of the starburst, almost all clouds are ejected from the galaxy as a result of the radiation pressure from the AGN, resulting in the formation of a quasar-like object. The origin of obscuration of AGNs is discussed with relevant observations.