Coevolution of Supermassive Black Holes and Circumnuclear Disks

We propose a new evolutionary model of a supermassive black hole (SMBH) and a circumnuclear disk (CND), taking into account the mass supply from a host galaxy and the physical states of the CND. In the model, two distinct accretion modes depending on the gravitational stability of the CND play a key role in accreting gas to a SMBH. (i) If the CND is gravitationally unstable, energy feedback from supernovae supports a geometrically thick, turbulent gas disk. The accretion in this mode is dominated by turbulent viscosity, and it is significantly larger than that in mode (ii), i.e., the CND is supported by gas pressure. Once the gas supply from the host is stopped, the high-accretion phase (~0.01-0.1 M☉ yr−1) changes to the low one (mode [ii], ~10−4 M☉ yr−1), but there is a delay of ~108 yr. Through this evolution, the gas-rich CND turns into the gas-poor stellar disk. We found that not all the gas supplied from the host galaxy to the central 100 pc region accretes onto the SMBH even in the high-accretion phase (mode [i]), because part of the gas is used to form stars. Outflow from the circumnuclear region also suppresses the growth of the SMBH. As a result, the final SMBH mass (MBH, final) is not proportional to the total gas mass supplied from the host galaxy (Msup); MBH, final/Msup decreases with Msup. This would indicate that it is difficult to form a SMBH with ~109 M☉ observed for high-z quasi-stellar objects. The evolution of the SMBH and CND would be related to the evolutionary tracks of different types of AGNs. We find that the AGN luminosity tightly correlates with the luminosity of the nuclear starburst only in the high-accretion phase (mode [ii]). This implies that the AGN-starburst connection depends on the evolution of the AGN activity.