A Population of Relic Intermediate-Mass Black Holes in the Halo of the
  Milky Way
  If "seed" central black holes were common in the subgalactic building blocks
that merged to form present-day massive galaxies, then relic intermediate-mass
black holes (IMBHs) should be present in the Galactic bulge and halo. We use a
particle tagging technique to dynamically populate the N-body Via Lactea II
high-resolution simulation with black holes, and assess the size, properties,
and detectability of the leftover population. The method assigns a black hole
to the most tightly bound central particle of each subhalo at infall according
to an extrapolation of the M_BH-sigma_* relation, and self-consistently follows
the accretion and disruption of Milky Way progenitor dwarfs and their holes in
a cosmological "live" host from high redshift to today. We show that, depending
on the minimum stellar velocity dispersion, sigma_m, below which central black
holes are assumed to be increasingly rare, as many as ~2000 (sigma_m=3 km/s) or
as few as ~70 (sigma_m=12 km/s) IMBHs may be left wandering in the halo of the
Milky Way today. The fraction of IMBHs kicked out of their host by
gravitational recoil is < 20%. We identify two main Galactic subpopulations,
"naked" IMBHs, whose host subhalos were totally destroyed after infall, and
"clothed" IMBHs residing in dark matter satellites that survived tidal
stripping. Naked IMBHs typically constitute 40-50% of the total and are more
centrally concentrated. We show that, in the sigma_m=12 km/s scenario, the
clusters of tightly bound stars that should accompany naked IMBHs would be
fainter than m_V=16 mag, spatially resolvable, and have proper motions of
0.1-10 milliarcsec per year. Their detection may provide an observational tool
to constrain the formation history of massive black holes in the early
Universe.
