Formation of Obscuring Walls by the Radiation Force from Circumnuclear Starbursts and the Implications for the Starburst–Active Galactic Nucleus Connection

Abstract

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 of a starburst as well as an AGN, in which the effects of the 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 because of the radiation pressure of a circumnuclear starburst. Next, in two-dimensional axisymmetric space, we analyze the configuration and the stability of geometrically thin walls that are balanced between radiation pressure and gravity. As a result, it is shown that the radiation force of 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 1011 L☉), there form double walls, the inner one of which is located between the nucleus and the circumnuclear starburst and the 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 mag for LSB/MSB 102 (L☉/M☉). As a consequence, double walls could heavily obscure the nucleus to make this a type 2 AGN. The outer wall may provide an explanation for the recent indications for large-scale obscuring materials in Seyfert 2 galaxies. 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 107 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 galaxies, whereas quasars are mostly observed as type 1 AGNs regardless of the star-forming activity in the host galaxies.