Obscuration of Active Galactic Nuclei by Circumnuclear Starbursts

We examine the possibility of the active galactic nucleus (AGN) obscuration by dusty gas clouds that spurt out from circumnuclear starburst regions. For the 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, it is shown that if the starburst is more luminous than the AGN, gas clouds are distributed extensively above a galactic disk with the assistance of radiation pressure from the starburst. The total covering factor of gas clouds reaches a maximum of around 20%. After several 10yr, gas clouds with larger optical depth form by cloud-cloud collisions and thereafter the clouds fall back due to 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 1s. On the other hand, if the AGN luminosity overwhelms that of the starburst, almost all clouds are ejected from the galaxy due to 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. Subject headings: galaxies: active — galaxies: nuclei — galaxies: Seyfert — galaxies: starburst — quasars: general — radiative transfer