Gaseous Dynamical Friction in Presence of Black Hole Radiative Feedback
  Dynamical friction is thought to be a principal mechanism responsible for
orbital evolution of massive black holes (MBHs) in the aftermath of galactic
mergers and an important channel for formation of gravitationally bound MBH
binaries. We use 2D radiative hydrodynamic simulations to investigate the
efficiency of dynamical friction in the presence of radiative feedback from an
MBH moving through a uniform density gas. We find that ionizing radiation that
emerges from the innermost parts of the MBH's accretion flow strongly affects
the dynamical friction wake and renders dynamical friction inefficient for a
range of physical scenarios. MBHs in this regime tend to experience positive
net acceleration, meaning that they speed up, contrary to the expectations for
gaseous dynamical friction in absence of radiative feedback. The magnitude of
this acceleration is however negligibly small and should not significantly
alter the velocity of MBHs over relevant physical timescales. Our results
suggest that suppression of dynamical friction is more severe at the lower mass
end of the MBH spectrum which, compounded with inefficiency of the gas drag for
lower mass objects in general, implies that $< 10^7$ solar mass MBHs have fewer
means to reach the centers of merged galaxies. These findings provide
formulation for a sub-resolution model of dynamical friction in presence of MBH
radiative feedback that can be easily implemented in large scale simulations.
