The role of galaxy mergers in the birth, pairing and merging of
supermassive black holes

I will review the status of models of the formation and merging of SMBHs in the
context of the current hierarchical structure formation model.  I will focus on
the rold of dissipational galaxy mergers as a key to understand formation,
merging and growth of SMBHs.  Galaxy mergers are known to deliver a large
fraction of the interstellar gas of galaxies towards the inner few hundred
parsecs.  However, for a long time direct calculations that model the
gravitational, hydrodynamical and radiative processes involved had only been
able to hint qualitatively at the phenomenon of meeger-driven gas inflows,
mostly due to their limited resolution in the nuclear region. I will discuss
the results of recent state-of-the art simulations which have allowed to
resolve scales of parsecs and below while the mergers of two galaxies takes
place. I will show how the gas brought towards the central hundred parsecs
produces a massive, star forming nuclear disk which compares well with recent
observations of merger remnants in the nearby Universe. The numerical
simulations demonstrate that a pair of supermassive black holes embedded in
such nuclear disk can bind into binary on very short timescales, less than a
million year. Whether or not the binary of supermassive black holes will always
be able to shrink down to the separation at which loss of energy via
gravitational waves becomes the dominant process in eroding its orbit is still
under investigation. Therefore the mapping between galaxy mergers and SMBHs is
still to be determined quantitatively, a task that should be accomplished
before the advent of gravitational wave experiments such as the Laser
Interferometer Space Antenna (LISA). Finally, in calculations probing
sub-parsec scales in the nuclear disk  more than a hundred million solar masses
of gas can be transported to the inner parsec via torques driven by spiral
instabilities. A massive black hole may form directly and rapidly from the
collapse of a central gas cloud produced by the inflow. Direct, fast massive
black hole formation may explain why bright quasars are seen to be in place
already less than a billion year after the Big Bang"