Super massive black holes in star forming gaseous circumnuclear discs
  Using N-body/SPH simulations we study the evolution of the separation of a
pair of SMBHs embedded in a star forming circumnuclear disk (CND). This type of
disk is expected to be formed in the central kilo parsec of the remnant of
gas-rich galaxy mergers. Our simulations indicate that orbital decay of the
SMBHs occurs more quickly when the mean density of the CND is higher, due to
increased dynamical friction. However, in simulations where the CND is
fragmented in high density gaseous clumps (clumpy CND), the orbits of the SMBHs
are erratically perturbed by the gravitational interaction with these clumps,
delaying, in some cases, the orbital decay of the SMBHs. The densities of these
gaseous clumps in our simulations and in recent studies of clumpy CNDs are
significantly higher than the observed density of molecular clouds in isolated
galaxies or ULIRGs, thus, we expect that SMBH orbits are perturbed less in real
CNDs than in the simulated CNDs of this study and other recent studies. We also
find that the migration timescale has a weak dependence on the star formation
rate of the CND. Furthermore, the migration timescale of a SMBH pair in a
star-forming clumpy CND is at most a factor three longer than the migration
timescale of a pair of SMBHs in a CND modeled with more simple gas physics.
Therefore, we estimate that the migration timescale of the SMBHs in a clumpy
CND is on the order of $10^7$ yrs.
