Dynamical friction ( DF ) against stars and gas is thought to be an important mechanism for orbital evolution of massive black holes ( MBHs ) in merger remnant galaxies .
Recent theoretical investigations however show that DF does not always lead to MBH inspiral .
For MBHs evolving in gas-rich backgrounds , the ionizing radiation that emerges from the innermost parts of their accretion flow can affect the surrounding gas in such a way to cause the MBHs to accelerate and gain orbital energy .
This effect was dubbed “ negative DF ” .
We use a semi-analytic model to study the impact of negative DF on pairs of MBHs in merger remnant galaxies evolving under the combined influence of stellar and gaseous DF .
Our results show that for a wide range of merger galaxy and MBH properties negative DF reduces the MBH pairing probability by \sim 40 \% .
The suppression of MBH pairing is most severe in galaxies with one or more of these properties : ( 1 ) a gas fraction of f _ { g } \geq 0.2 ; ( 2 ) a galactic gas disk rotating close to the circular velocity ; ( 3 ) MBH pairs in prograde , low eccentricity orbits , and ( 4 ) MBH pairs with mass < { few } \times 10 ^ { 7 } M _ { \odot } .
The last point is of importance because MBH pairs in this mass range are direct progenitors of merging binaries targeted by the future space-based gravitational wave observatory LISA .