Stellar streams from black hole-rich star clusters

ABSTRACT Nearly a hundred progenitor-less, thin stellar streams have been discovered in the Milky Way, thanks to Gaia and related surveys. Most streams are believed to have formed from star clusters and it was recently proposed that extended star clusters – rich in stellar-mass black holes (BHs) – are efficient in creating streams. To better understand the nature of stream progenitors, we quantify the differences between streams originating from star clusters with and without BHs using direct N-body models and a new model for the density profiles of streams based on time-dependent escape rates from clusters: the Quantifying Stream Growth (QSG) model. QSG facilitates the rapid exploration of parameter space and provides an analytic framework to understand the impact of different star cluster properties and escape conditions on the structure of streams. Using these models it is found that, compared to streams from BH-free clusters on the same orbit, streams of BH-rich clusters: (1) are approximately 5 times more massive; (2) have a peak density 3 times closer to the cluster $1~{\rm Gyr}$ post-evaporation (for orbits of Galactocentric radius $\gtrsim 10~{\rm kpc}$), and (3) have narrower peaks and more extended wings in their density profiles. We discuss other observable stream properties that are affected by the presence of BHs in their progenitor cluster, namely the width of the stream, its radial offset from the orbit, and the properties of the gap at the progenitor’s location. Our results provide a step towards using stellar streams to constrain the BH content of evaporated (globular) star clusters.