Canaries in a Coal Mine: Using Globular Clusters to Place Limits on Massive Black Holes in the Galactic Halo

Abstract

We explore the possibility that massive black holes comprise a significant fraction of the dark matter of our galaxy by studying the dissolution of galactic globular clusters bombarded by them. In our simulations, we evolve the clusters along a sequence of King models determined by changes of state resulting from collisions with the black holes. The results divide naturally into regimes of `small' and `large' black hole mass. `Small' black holes do not destroy clusters in single collisions; their effect is primarily cumulative, leading to a relation between $\mbh$ and $\fhalo$, the fraction of the halo in black holes of mass $\mbh$, which is $\fhalo\mbh < $ constant (up to logarithmic corrections). For $\fhalo=1$, we find $\mbh \simless 10^{3} \msun$ by requiring survival of the same clusters studied by Moore (1993), who neglected cluster evolution, mass loss, and stochasticity of energy inputs in his estimates, but reached a similar conclusion. `Large' black holes may not penetrate a cluster without disrupting it; their effect is mainly catastrophic (close collisions), but also partly cumulative (distant collisions). In the large $\mbh$ limit, $\fhalo$ (but not $\mbh$) can be constrained by computing the probability that a cluster survives a combination of close, destructive encounters and distant, nondestructive encounters. We find that it is unlikely that $\fhalo \simgreat 0.3$ by requiring 50 per cent survival probability for Moore's clusters over $10^{10}$ years.