Pairing of Massive Black Holes in Merger Galaxies Driven by Dynamical
  Friction
  Motivated by observational searches for massive black hole (MBH) pairs at
kiloparsec separations we develop a semi-analytic model to describe their
orbital evolution under the influence of stellar and gaseous dynamical friction
(DF). The goal of this study is to determine how the properties of the merger
remnant galaxy and the MBHs affect the likelihood and timescale for formation
of a close MBH pair with separation of < 1 pc. We compute approximately 40,000
configurations that cover a wide range of host galaxy properties and
investigate their impact on the orbital evolution of unequal mass MBH pairs. We
find that the percentage for MBH pairing within a Hubble time is larger than
80% in remnant galaxies with a gas fraction < 20% and in galaxies hosting MBH
pairs with total mass > 10^6 solar mass and mass ratios > 1/4. Among these, the
remnant galaxies characterized by the fastest formation of close,
gravitationally bound MBHs have one or more of the following properties: (1)
large stellar bulge, (2) comparable mass MBHs and (3) a galactic gas disk
rotating close to the circular speed. In such galaxies, the MBHs with the
shortest inspiral times, which are likely progenitors of coalescing MBHs, are
either on circular prograde orbits or on very eccentric retrograde orbits. Our
model also indicates that remnant galaxies with opposite properties, that host
slowly evolving MBH pairs, are the most likely hosts of dual AGNs at kiloparsec
separations.
