Formation of obscuring walls by the radiation force from circumnuclear starbursts and the implications for the starburst-active galactic nucleus connection

We explore the formation of dusty gas walls induced by a circumnuclear star burst 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 wh ich the effects of the optical depth of dusty gas are taken into considerat ion. First, we solve the hydrostatic equations in spherical symmetry couple d with the frequency-dependent radiative processes to demonstrate that a ge ometrically thin, optically thick wall forms because of the radiation press ure of a circumnuclear starburst. Next, in two-dimensional axisymmetric spa ce, we analyze the configuration and the stability of geometrically thin wa lls that are balanced between radiation pressure and gravity. As a result, it is shown that the radiation force of the circumnuclear starburst works t o stabilize optically thick walls surrounding the nucleus. In the case of a brighter starburst with a fainter AGN (e.g., L-SB/M-SB greater than or sim ilar to 10 [L-./M-.] and L-AGN less than or similar to 10(11) L-.), there f orm 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 A(v) similar to 10 mag for L-SB/M-SB greater than or similar to 10(2) (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 l arge-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 10(7) yr owing to the disappearance of walls. In contrast, if the AGN itself is muc h brighter than the starburst, as a quasar is, then neither wall forms rega rdless of the starburst activity, and the nucleus is likely to be identifie d as type 1. To conclude, the radiatively supported gas walls could be resp onsible for the putative correlation between AGN type and the starbursts, w hereby Seyfert 2 galaxies are more frequently associated with circumnuclear starbursts than type 1 galaxies, whereas quasars are mostly observed as ty pe 1 AGNs regardless of the star-forming activity in the host galaxies.