Formation of Large-Scale Obscuring Wall and Active Galactic Nucleus Evolution Regulated by Circumnuclear Starbursts

Abstract

By considering the radiative force by a circumnuclear starburst as well as an active galactic nucleus (AGN), we analyze the equilibrium configuration and the stability of dusty gas in the circumnuclear regions. It is found that the radiative force by an intensive starburst can support a stable gaseous wall with a scale height of several hundred parsecs. Moreover, by taking the simple stellar evolution in the starburst into account, we find that the covering factor of the wall decreases on a timescale of several times 107 yr. The large-scale wall, if formed, works to obscure the nucleus as a result of the dust opacity. Hence, it is anticipated that the index of AGN type tends to shift from higher to lower in several times 107 yr as the circumnuclear starburst becomes dimmer. On the other hand, if the AGN itself is brighter than the circumnuclear starburst (e.g., the quasar case), no stable large-scale wall forms. In that case, the AGN is most probably identified as type 1. The present mechanism may provide a physical explanation for the putative correlation between AGN type and host properties whereby Seyfert type 2 galaxies are more frequently associated with circumnuclear starbursts than type 1 galaxies, whereas quasars are mostly observed as type 1 regardless of star-forming activity in the host galaxies.