Gaseous dynamical friction under radiative feedback: do intermediate-mass black holes speed up or down?

Abstract

ABSTRACT Coalescence of intermediate-mass black holes (IMBHs) as a result of the migration toward galactic centres 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 104-M⊙ BH moving at the velocity vflow through the homogeneous medium with metallicity Z in the range of 0–0.1 Z⊙ and density n∞. We show that, if n∞ ≲ 106 cm−3 and vflow ≲ 60 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∞ ≳ 106 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 time-scale of ∼0.01 Myr, much shorter than the dynamical time-scale in galactic discs. Our results suggest that IMBHs that encounter the dense clouds rapidly migrate toward galactic centres, where they possibly coalescence with others.