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.