A supra-massive population of stellar-mass black holes in the globular cluster Palomar 5

TL;DR

This paper proposes that a large population of stellar-mass black holes in Palomar 5 explains its extended tidal tails and predicts the cluster will eventually dissolve into a black hole-dominated remnant, highlighting black holes' role in cluster evolution.

Contribution

It introduces a model where black holes significantly influence the formation and evolution of Palomar 5's tidal features, a novel explanation for its observed properties.

Findings

Black hole population accounts for Palomar 5's tidal tails.

Cluster will dissolve into a black hole-dominated remnant.

Black hole-rich clusters may form thin stellar streams.

Abstract

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.