Bounds on the Mass and Abundance of Dark Compact Objects and Black Holes in Dwarf Spheroidal Galaxy Halos

We establish new dynamical constraints on the mass and abundance of compact objects in the halo of dwarf spheroidal (dSph) galaxies. In order to preserve kinematically cold the second peak of the Ursa Minor (UMi) dSph against gravitational scattering, we place upper limits on the density of compact objects as a function of their assumed mass. The mass of the dark matter constituents cannot be larger than 103 M☉ at a halo density in UMi's core of 0.35 M☉ pc-3. This constraint rules out a scenario in which dark halo cores are formed by two-body relaxation processes. Our bounds on the fraction of dark matter in compact objects with masses ≳3 × 103 M☉ improve those based on dynamical arguments in the Galactic halo. In particular, objects with masses ~105 M☉ can comprise no more than a halo mass fraction ~0.01. Better determinations of the velocity dispersion of old overdense regions in dSph galaxies may result in more stringent constraints on the mass of halo objects. For illustration, if the preliminary value of 0.5 km s-1 for the secondary peak of UMi is confirmed, compact objects with masses above ~100 M☉ could be excluded from comprising all its dark matter halo.