  Coalescence of intermediate-mass black holes (IMBHs) as a result of the
migration toward galactic centers via dynamical friction may contribute to the
formation of supermassive BHs. Here we reinvestigate the gaseous dynamical
friction, which was claimed to be inefficient with radiative feedback from BHs
in literature, by performing 3D radiation-hydrodynamics simulations that solve
the flow structure in the vicinity of BHs. We consider a $10^4~M_\odot$ BH
moving at the velocity $V_{\rm flow}$ through the homogeneous medium with
metallicity $Z$ in the range of $0-0.1~Z_\odot$ and density $n_{\infty}$. We
show that, if $n_{\infty} \lesssim 10^{6}~{\rm cm^{-3}}$ and $V_{\rm flow}
\lesssim 60~{\rm km~s^{-1}}$, the BH is accelerated forward because of the
gravitational pull from a dense shell ahead of an ionized bubble around the BH,
regardless of the value of $Z$. If $n_{\infty} \gtrsim 10^{6}~{\rm cm^{-3}}$,
however, our simulation shows the opposite result. The ionized bubble and
associating shell temporarily appear, but immediately go downstream with
significant ram pressure of the flow. They eventually converge into a massive
downstream wake, which gravitationally drags the BH backward. The BH
decelerates over the timescale of $\sim 0.01$~Myr, much shorter than the
dynamical timescale in galactic disks. Our results suggest that IMBHs that
encounter the dense clouds rapidly migrate toward galactic centers, where they
possibly coalescence with others.
