A stellar-mass black hole population in the globular cluster NGC 6101?

TL;DR

This study uses N-body simulations to investigate the apparent lack of mass segregation in NGC 6101, suggesting a significant black hole population influences its dynamical state and observational features.

Contribution

It demonstrates that high black hole retention fractions can mask mass segregation signatures in globular clusters like NGC 6101.

Findings

High black hole retention fractions (>50%) can hide mass segregation.

NGC 6101 likely formed with a large initial radius in a satellite galaxy.

Black hole kick velocities are inferred to be lower than neutron star kicks.

Abstract

Dalessandro et al. observed a similar distribution for blue straggler stars and main-sequence turn-off stars in the Galactic globular cluster NGC 6101, and interpreted this feature as an indication that this cluster is not mass-segregated. Using direct N-body simulations, we find that a significant amount of mass segregation is expected for a cluster with the mass, radius and age of NGC 6101. Therefore, the absence of mass segregation cannot be explained by the argument that the cluster is not yet dynamically evolved. By varying the retention fraction of stellar-mass black holes, we show that segregation is not observable in clusters with a high black hole retention fraction (>50% after supernova kicks and >50% after dynamical evolution). Yet all model clusters have the same amount of mass segregation in terms of the decline of the mean mass of stars and remnants with distance to the centre. We also discuss how kinematics can be used to further constrain the presence of a stellar-mass black hole population and distinguish it from the effect of an intermediate-mass black hole. Our results imply that the kick velocities of black holes are lower than those of neutron stars. The large retention fraction during its dynamical evolution can be explained if NGC 6101 formed with a large initial radius in a Milky Way satellite.