The Effect of Gaseous Dissipation on the Fate of Supermassive Black Holes in Merging Galaxies

Abstract

We analyze the effect of dissipation on the orbital evolution of supermassive black holes (SMBHs) using high-resolution gasdynamical simulations of binary equal- and unequal-mass mergers of disk dominated galaxies that include the effects of radiative cooling and star formation. We find that equal-mass mergers always lead to the formation of a close SMBH pair at the center of the remnant with separations limited solely by the adopted force resolution of ~ 100pc. Instead, the final separation of the SMBHs in unequal-mass mergers depends sensitively on how the central structure of the merging galaxies is modified by dissipation. In the absence of dissipation, the companion galaxy is entirely disrupted before the merger is completed leaving its SMBH wandering at a distance too far from the center of the remnant for the formation of a close pair. In contrast, gas cooling facilitates the pairing process by increasing the resilience of the companion galaxy to tidal disruption. Our results suggest that semi-analytic models of hierarchical SMBH growth that neglect the effect of dissipation likely overestimate the number of wandering SMBHs in massive galaxies. The large gas inflows associated with the strong tidal torques during the merger lead to the formation of massive, rotationally supported nuclear disks with sizes, masses and rotational velocities similar to the ones observed spectroscopically for few AGNs and ULRIGs. These disks are gravitationally unstable and likely provide the necessary fuel for feeding the SMBHs and bridging the gap between the large scale flows and the viscous accretion taking place once the gas has reached the AGN accretion disk at subparsec scales.