Massive Black Holes in Dwarf Spheroidal Galaxy Haloes?

Abstract

It is now established that several of the Local Group dwarf Spheroidal galaxies (dSphs) have large mass-to-light ratios. We consider the possibility that the dark matter in the halos of dSphs is composed of massive black holes with masses in the range 10^5 to 10^7 solar masses. We use direct N-body simulations to determine the long term evolution of a two-component dSph composed of a pressure-supported stellar population in a black hole halo. The black holes are initially distributed according to a Navarro, Frenk & White profile. For black hole masses between 10^5 and 10^6 solar masses, the dark matter halo evolves towards a shallower inner profile in less than a Hubble time. This suggests that black holes in this mass range might provide an explanation for the origin of the dark matter cores inferred from observations of Low Surface Brightness galaxy rotation curves. We compare the simulated evolution of the stellar population with observed data for the Draco dSph. The dependence of the heating rate on the black hole mass is determined, and an upper limit of 10^5 solar masses is placed on the individual black holes comprising the dark matter halo of Draco, if its present stellar distribution is representative of the initial one. We also present a simple scaling argument which demonstrates that the dynamical heating of an initially compact, though not self-gravitating, stellar distribution might produce a remnant distribution similar in extent to Draco after 10 Gyr, even for black hole masses somewhat in excess of 10^5 solar masses.