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 supermassiveblack holes (SMBHs) using high-resolution self-consistent gasdynamicalsimulations of binary equal- and unequal-mass mergers of disk galaxies. Thegalaxy models are consistent with the LCDM paradigm of structure formation andthe simulations include the effects of radiative cooling and star formation. Wefind that equal-mass mergers always lead to the formation of a close SMBH pairat the center of the remnant with separations limited solely by the adoptedforce resolution of ~ 100 pc. Instead, the final SMBH separation inunequal-mass mergers depends sensitively on how the central structure of themerging galaxies is modified by dissipation. In the absence of dissipation, thesatellite galaxy can be entirely disrupted before the merger is completedleaving its SMBH wandering at a distance too far from the center of the remnantfor the formation of a close pair. In contrast, we show that gas coolingfacilitates the pairing process by increasing the resilience of the companiongalaxy to tidal disruption. Moreover, we demonstrate that merging disk galaxiesconstructed to obey the M(BH)-sigma relation, move relative to it depending onwhether they undergo a dissipational or collisionless merger, regardless of themass ratio of the merging systems. Collisionless simulations reveal thatremnants tend to move away from the mean relation highlighting the role ofgas-poor mergers as a possible source of scatter. In dissipational mergers, the interplay between strong gas inflows associatedwith the formation of massive nuclear disks and the consumption of gas by starformation provides the necessary fuel to the SMBHs and allows the mergerremnants to satisfy the relation.Comment: Accepted for publication in ApJL, 5 pages, 3 figures, LaTeX (uses emulateapj5.sty). Conclusions unchanged, new simulations performed, M(BH)-sigma plot update