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° 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 billion years 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.

Palomar 5 是银河晕中最稀疏的星团之一，以其壮观的潮汐尾迹而闻名，它横跨天空超过 20°。与N-体模拟，我们表明这两个显着特征都可能来自恒星质量的黑洞种群，占当今星团质量的约 20%。在这种情况下，Palomar 5 形成的“正常”黑洞质量分数只有百分之几，但恒星的流失率高于黑洞，因此​​黑洞分数逐渐增加。这使星团膨胀，增强了潮汐剥离和尾巴的形成。从现在开始的 10 亿年后，该星团将溶解为 100% 的黑洞星团。最初更密集的星团最终以更低的黑洞分数、更小的尺寸和没有可观察到的尾巴结束。因此，以黑洞为主的扩展星团可能是最近在银河晕中发现的稀薄恒星流的前身。