WANDERING BLACK HOLES IN BRIGHT DISK GALAXY HALOS

We perform SPH+N-body cosmological simulations of massive disk galaxies, including a formalism for black hole (BH) seed formation and growth, and find that satellite galaxies containing supermassive BH seeds are often stripped as they merge with the primary galaxy. These events naturally create a population of “wandering” BHs 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 BH 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 BHs. Provided that these wandering BHs 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.