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 self-consistent gasdynamical simulations of binary equal- and unequal-mass mergers of disk galaxies. The galaxy models are consistent with the ΛCDM 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 SMBH separation 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 satellite galaxy can be 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. Moreover, we demonstrate that merging disk galaxies constructed to obey the M_BH-σ relation move relative to it depending on whether they undergo a dissipational or collisionless merger, regardless of the mass ratio of the merging systems. Collisionless simulations reveal that remnants tend to move away from the mean relation, highlighting the role of gas-poor mergers as a possible source of scatter. 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 provides the necessary fuel to the SMBHs and allows the merger remnants to satisfy the relation