Wandering Black Holes in Bright Disk Galaxy Halos

TL;DR

This study uses cosmological simulations to show that galaxy mergers produce wandering black holes in galaxy halos, which could explain some off-nuclear ultraluminous X-ray sources.

Contribution

It introduces a formalism for black hole seed formation in simulations and demonstrates the creation of wandering black holes from satellite galaxy stripping.

Findings

Galaxies like the Milky Way may host 5-15 wandering black holes.

Satellite dwarf galaxies can harbor black hole seeds similar to the Magellanic Clouds.

Wandering black holes with accretion disks could explain off-nuclear X-ray sources like HLX-1.

Abstract

We perform SPH+N-body cosmological simulations of massive disk galaxies, including a formalism for black hole seed formation and growth, and find that satellite galaxies containing supermassive black hole seeds are often stripped as they merge with the primary galaxy. These events naturally create a population of "wandering" black holes that are the remnants of stripped satellite cores; galaxies like the Milky Way may host 5 -- 15 of these objects within their halos. The satellites that harbor black hole seeds are comparable to Local Group dwarf galaxies such as the Small and Large Magellanic Clouds; these galaxies are promising candidates to host nearby intermediate mass black holes. Provided that these wandering black holes retain a gaseous accretion disk from their host dwarf galaxy, they give a physical explanation for the origin and observed properties of some recently discovered off-nuclear ultraluminous X-ray sources such as HLX-1.