  Palomar 5 is one of the sparsest star clusters in the Galactic halo and is
best-known for its spectacular tidal tails, spanning over 20 degrees across the
sky. With N-body simulations we show that both distinguishing features can
result from a stellar-mass black hole population, comprising ~20% of the
present-day cluster mass. In this scenario, Palomar 5 formed with a `normal'
black hole mass fraction of a few per cent, but stars were lost at a higher
rate than black holes, such that the black hole fraction gradually increased.
This inflated the cluster, enhancing tidal stripping and tail formation. A
gigayear from now, the cluster will dissolve as a 100% black hole cluster.
Initially denser clusters end up with lower black hole fractions, smaller
sizes, and no observable tails. Black hole-dominated, extended star clusters
are therefore the likely progenitors of the recently discovered thin stellar
streams in the Galactic halo.
