  We examine the pairing process of supermassive black holes (SMBHs) down to
scales of 20-100 pc using a set of N-body/SPH simulations of binary mergers of
disk galaxies with mass ratios of 1:4 and 1:10. Our numerical experiments are
designed to represent merger events occurring at various cosmic epochs. The
initial conditions of the encounters are consistent with the LambdaCDM paradigm
of structure formation, and the simulations include the effects of radiative
cooling, star formation, and supernovae feedback. We find that the pairing of
SMBHs depends sensitively on the amount of baryonic mass preserved in the
center of the companion galaxies during the last phases of the merger. In
particular, due to the combination of gasdynamics and star formation, we find
that a pair of SMBHs can form in 1:10 minor mergers provided that galaxies are
relatively gas-rich (gas fractions of 30% of the disk mass) and that the
mergers occur at relatively high redshift (z~3), when dynamical friction
timescales are shorter. Since 1:10 mergers are most common events during the
assembly of galaxies, and mergers are more frequent at high redshift when
galaxies are also more gas-rich, our results have positive implications for
future gravitational wave experiments such as the Laser Interferometer Space
Antenna.
