The Origins of Off-Centre Massive Black Holes in Dwarf Galaxies

TL;DR

This study uses cosmological simulations to show that off-center massive black holes in dwarf galaxies are common due to galaxy mergers and are unlikely to migrate to the center within a Hubble time, making them difficult to detect.

Contribution

It demonstrates that galaxy mergers primarily cause off-center black holes in dwarfs and that these black holes rarely sink to the center within the age of the universe.

Findings

Off-center MBHs are common in dwarf galaxies.

Dynamical timescales prevent MBHs from reaching the galaxy center within a Hubble time.

Detecting wandering MBHs electromagnetically is extremely challenging.

Abstract

Massive black holes often exist within dwarf galaxies, and both simulations and observations have shown that a substantial fraction of these may be off-center with respect to their hosts. We trace the evolution of off-center massive black holes (MBHs) in dwarf galaxies using cosmological hydrodynamical simulations, and show that the reason for off-center locations is mainly due to galaxy-galaxy mergers. We calculate dynamical timescales and show that off-center MBHs are unlikely to sink to their galaxys' centers within a Hubble time, due to the shape of the hosts' potential wells and low stellar densities. These wandering MBHs are unlikely to be detected electromagnetically, nor is there a measurable dynamical effect on the galaxy's stellar population. We conclude that off-center MBHs may be common in dwarfs, especially if the mass of the MBH is small or the stellar mass of the host galaxy is large. However detecting them is extremely challenging, because their accretion luminosities are very low and they do not measurably alter the dynamics of their host galaxies.