Formation and Hardening of Supermassive Black Hole Binaries in Minor
  Mergers of Disk Galaxies
  We model for the first time the complete orbital evolution of a pair of
Supermassive Black Holes (SMBHs) in a 1:10 galaxy merger of two disk dominated
gas-rich galaxies, from the stage prior to the formation of the binary up to
the onset of gravitational wave emission when the binary separation has shrunk
to 1 milli parsec. The high-resolution smoothed particle hydrodynamics (SPH)
simulations used for the first phase of the evolution include star formation,
accretion onto the SMBHs as well as feedback from supernovae explosions and
radiative heating from the SMBHs themselves. Using the direct N-body code
\phi-GPU we evolve the system further without including the effect of gas,
which has been mostly consumed by star formation in the meantime. We start at
the time when the separation between two SMBHs is ~ 700 pc and the two black
holes are still embedded in their galaxy cusps. We use 3 million particles to
study the formation and evolution of the SMBH binary till it becomes hard.
After a hard binary is formed, we reduce (reselect) the particles to 1.15
million and follow the subsequent shrinking of the SMBH binary due to 3-body
encounters with the stars. We find approximately constant hardening rates and
that the SMBH binary rapidly develops a high eccentricity. Similar hardening
rates and eccentricity values are reported in earlier studies of SMBH binary
evolution in the merging of dissipation-less spherical galaxy models. The
estimated coalescence time is ~ 2.9 Gyr, significantly smaller than a Hubble
time. We discuss why this timescale should be regarded as an upper limit. Since
1:10 mergers are among the most common interaction events for galaxies at all
cosmic epochs, we argue that several SMBH binaries should be detected with
currently planned space-borne gravitational wave interferometers, whose
sensitivity will be especially high for SMBHs in the mass range considered
here.
