Formation of Obscuring Walls by Radiation Force from Circumnuclear Starbursts and Implications for Starburst-active Galactic Nucleus Connection

We explore the formation of dusty gas walls induced by a circumnuclear starburst around an active galactic nucleus (AGN). We concentrate our attention on the role of the radiation force by a starburst as well as an AGN, where the effects of optical depth of dusty gas are taken into consideration. First, we solve the hydrostatic equations in spherical symmetry coupled with the frequency-dependent radiative processes, to demonstrate that a geometrically thin, optically thick wall forms due to the radiation pressure by a circumnuclear starburst. Next, in two-dimensional axisymmetric space, we analyze the configuration and the stability of geometrically thin walls which are in balance between radiation pressure and gravity. As a result, it is shown that the radiation force by the circumnuclear starburst works to stabilize optically thick walls surrounding the nucleus. In the case of a brighter starburst with a fainter AGN (e.g. LSB/MSB> ∼ 10[L⊙/M⊙] and LAGN < ∼ 10L⊙), there form double walls, an inner one of which is located between the nucleus and the circumnuclear starburst, and an outer one of which enshrouds both the starburst regions and the nucleus. The total extinction of both walls turns out to be larger for a brighter starburst, which is AV ∼ 10 for LSB/MSB> ∼ 10[L⊙/M⊙]. As a consequence, double walls could heavily obscure the nucleus to make the AGN type 2. The outer wall may provide an explanation for the recent indications for large-scale obscuring materials in Seyfert 2’s. Also, it is predicted that the AGN type is time-dependent according to the stellar evolution in the starburst, which shifts from type 2 to type 1 in several times 10 yr owing to the disappearance of walls. In contrast, if the AGN itself is much brighter than the starburst as a quasar is, then neither wall forms regardless of the starburst activity and the nucleus is likely to be identified as type 1. To conclude, the radiatively-supported gas walls could be responsible for the putative correlation between AGN type and the starbursts, whereby Seyfert 2 galaxies are more frequently associated with circumnuclear starbursts than type 1, whereas quasars are mostly observed as type 1 regardless of star-forming activity in the host galaxies. Subject headings: galaxies: active — galaxies: evolution — galaxies: nuclei — galaxies: starburst — quasars: general — radiative transfer