Subsonic accretion and dynamical friction for a black hole moving through a self-interacting scalar dark matter cloud

TL;DR

This paper analyzes the flow of self-interacting scalar dark matter around a moving black hole, revealing how accretion and dynamical friction differ from classical models, with implications for understanding dark matter interactions.

Contribution

It provides analytical insights into scalar dark matter behavior near black holes, highlighting differences in accretion rates and dynamical friction compared to collisionless and perfect gas models.

Findings

Accretion rate exceeds collisionless particle case by c/c_s.

Dynamical friction is reduced compared to perfect gas and Chandrasekhar's result.

Scalar dark matter behaves as a perfect gas with adiabatic index 2 at large radii.

Abstract

We investigate the flow around a black hole moving through a cloud of self-interacting scalar dark matter. We focus on the large scalar mass limit, with quartic self-interactions, and on the subsonic regime. We show how the scalar field behaves as a perfect gas of adiabatic index $\gamma_{\rm ad}=2$ at large radii while the accretion rate is governed by the relativistic regime close to the Schwarzschild radius. We obtain analytical results thanks to large-radius expansions, which are also related to the small-scale relativistic accretion rate. We find that the accretion rate is greater than for collisionless particles, by a factor $c/c_s \gg 1$, but smaller than for a perfect gas, by a factor $c_s/c \ll 1$, where $c_s$ is the speed of sound. The dynamical friction is smaller than for a perfect gas, by the same factor $c_s/c \ll 1$, and also smaller than Chandrasekhar's result for collisionless particles, by a factor $c_s/(cC)$, where $C$ is the Coulomb logarithm. It is also smaller than for fuzzy dark matter, by a factor $v_0/c \ll 1$.