We establish constraints on the mass and abundance of black holes in the Galactic halo by determining their impact on globular clusters which are conventionally considered to be little evolved .
Using detailed Monte Carlo simulations , and simple evolutionary models , we argue that black holes with masses M _ { bh } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2. % 0 pt \hbox { $ \mathchar 318 $ } } ( 1 - 3 ) \times 10 ^ { 6 } { M _ { \odot } } can comprise no more than a fraction f _ { bh } \approx 0.17 of the total halo density at Galactocentric radius R \approx 8 kpc .
This bound arises from requiring stability of the cluster mass function .
A more restrictive bound may be derived if we demand that the probability of destruction of any given , low mass ( M _ { c } \approx ( 2.5 - 7.5 ) \times 10 ^ { 4 } { M _ { \odot } } ) globular cluster not exceed 50 % ; this bound is f _ { bh } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2. % 0 pt \hbox { $ \mathchar 316 $ } } 0.025 - 0.5 at R \approx 8 kpc .
This constraint improves those based on disk heating and dynamical friction arguments as well as current lensing results .
At smaller radius , the constraint on f _ { bh } strengthens , while , at larger radius , an increased fraction of black holes is allowed .