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 \lesssim 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 } M _ { \odot } and mass ratios \geq 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 .