Wandering Supermassive Black Holes in Milky-Way-mass Halos

We present a self-consistent prediction from a large-scale cosmological simulation for the population of "wandering" supermassive black holes (SMBHs) of mass greater than 10⁶ M _o on long-lived, kpc-scale orbits within Milky Way (MW)-mass galaxies. We extract a sample of MW-mass halos from the Romulus25 cosmological simulation, which is uniquely able to capture the orbital evolution of SMBHs during and following galaxy mergers. We predict that such halos, regardless of recent merger history or morphology, host an average of 5.1 3.3 SMBHs, including their central black hole, within 10 kpc from the galactic center and an average of 12.2 8.4 SMBHs total within their virial radius, not counting those in satellite halos. Wandering SMBHs exist within their host galaxies for several Gyr, often accreted by their host halo in the early Universe. We find, with >4σ significance, that wandering SMBHs are preferentially found outside of galactic disks.