Supersonic friction of a black hole traversing a self-interacting scalar dark matter cloud

Understand

Black Holes (BH) traversing a dark matter cloud made out of a self-interacting scalar soliton are slowed down by two complementary effects.

• At low subsonic speeds, the BH accretes dark matter and this is the only source of dragging along its motion, if we neglect the backreaction of the cloud self-gravity.
• The situation changes at larger supersonic speeds where a shock appears.
• This leads to the emergence of an additional friction term, associated with the gravitational and scalar pressure interactions and with the wake behind the moving BH.