  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 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 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, we show that 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)-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. 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.
