Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback. III. Enhanced Luminosity of Intermediate Mass Black Holes Moving at Supersonic Speeds

TL;DR

This study investigates how the luminosity and accretion rates of intermediate mass black holes are affected by their supersonic motion through surrounding media, revealing conditions that enhance their growth and detectability.

Contribution

It introduces a comprehensive analytical model and simulation results showing increased accretion rates for moving black holes due to radiative feedback effects.

Findings

Accretion rate peaks at BH velocities around 50 km/s.

Enhanced accretion occurs due to bow-shock formation and ionization front dynamics.

Moving BHs can grow faster in certain interstellar conditions.

Abstract

In this third paper of a series, we study the growth and luminosity of black holes (BHs) in motion with respect to their surrounding medium. We run a large set of two-dimensional (2D) axis-symmetric simulations to explore a large parameter space of initial conditions and formulate an analytical model for the accretion. Contrary to the case without radiation feedback, the accretion rate increases with increasing BH velocity $\vbh$ reaching a maximum value at $\vbh=2c_{\rm s,in}\sim 50$ km/s, where $c_{\rm s,in}$ is the sound speed inside the "cometary-shaped" \hii region around the BH, before decreasing as $\vbh^{-3}$ when the I-front becomes {\it R}-type (rarefied) and the accretion rate approaches the classical Bondi-Hoyle-Lyttleton solution. The increase of the accretion rate with $\vbh$ is produced by the formation of a {\it D}-type (dense) ionization front (I-front) preceded by a standing bow-shock that reduces the downstream gas velocity to transonic values. There is a range of densities and velocities where the dense shell is unstable producing periodic accretion rate peaks which can significantly increase the detectability of intermediate mass BHs. We find that the mean accretion rate for a moving BH is larger than that of a stationary BH of the same mass if the medium temperature is $T_\infty<10^4$ K. This result could be important for the growth of seed BHs in the multi-phase medium of the first galaxies and for building and early X-ray background that may affect the formation of the first galaxies and the reionization process.