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 $10^6$ M$_{\odot}$ 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 (Tremmel et al. 2017), 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 $5.1 \pm 3.3$ SMBHs, including their central black hole,
within 10 kpc from the galactic center and an average of $12.2 \pm 8.4$ SMBHs
total within their virial radius, not counting those in satellite halos.
Wandering SMBHs exist within their host galaxies for several Gyrs, often
accreted by their host halo in the early Universe. We find, with $>4\sigma$
significance, that wandering SMBHs are preferentially found outside of galactic
disks.
