If massive black holes (BHs) are ubiquitous in galaxies and galaxies
experience multiple mergers during their cosmic assembly, then BH binaries
should be common albeit temporary features of most galactic bulges.
Observationally, the paucity of active BH pairs points toward binary lifetimes
far shorter than the Hubble time, indicating rapid inspiral of the BHs down to
the domain where gravitational waves lead to their coalescence. Here, we review
a series of studies on the dynamics of massive BHs in gas-rich galaxy mergers
that underscore the vital role played by a cool, gaseous component in promoting
the rapid formation of the BH binary. The BH binary is found to reside at the
center of a massive self-gravitating nuclear disc resulting from the collision
of the two gaseous discs present in the mother galaxies. Hardening by
gravitational torques against gas in this grand disc is found to continue down
to sub-parsec scales. The eccentricity decreases with time to zero and when the
binary is circular, accretion sets in around the two BHs. When this occurs,
each BH is endowed with it own small-size (< 0.01 pc) accretion disc comprising
a few percent of the BH mass. Double AGN activity is expected to occur on an
estimated timescale of < 1 Myr. The double nuclear point-like sources that may
appear have typical separation of < 10 pc, and are likely to be embedded in the
still ongoing starburst. We note that a potential threat of binary stalling, in
a gaseous environment, may come from radiation and/or mechanical energy
injections by the BHs. Only short-lived or sub-Eddington accretion episodes can
guarantee the persistence of a dense cool gas structure around the binary
necessary for continuing BH inspiral.Comment: To appear in "2007 STScI Spring Symposium: Black Holes", eds. M.
Livio & A. M. Koekemoer, Cambridge University Press, 25 pages, 12 figure