The Fate of Supermassive Black Holes and the Evolution of the M BH -σ Relation in Merging Galaxies: The Effect of Gaseous Dissipation

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 galaxies. The initial conditions are consistent with the LCDM paradigm of structure formation and the simulations 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 ~ 100 pc. 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 the satellite 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. We demonstrate that galaxies constructed to obey the M(BH)-sigma relation, move relative to it depending on whether they undergo a dissipational or collisionless merger, regardless of the mass ratio of the merging systems. In dissipational mergers, the interplay between strong gas inflows associated with the formation of massive nuclear disks and the consumption of gas by star formation may provide the necessary fuel to the SMBHs and allow their host galaxies to satisfy the relation.