Effects of stellar and intermediate-mass black holes on the degree of energy equipartition in globular clusters

Stellar encounters drive the dynamical evolution of globular clusters (GCs) in a process known as two-body relaxation. The kinetic energy exchange during the stellar encounters makes the cluster evolve towards a partial degree of energy equipartition, which introduces an observable mass-dependent velocity dispersion. Motivated by current state-of-the-art observations of GCs, we analyse the degree of energy equipartition of a sample of 100 numerical simulations of GCs hosting either a stellar-mass black holes system (BHS), an intermediate-mass black hole (IMBH) or neither of them. For the first time, we systematically explore the signatures that the presence of BHS or IMBHs produces on the degree of energy equipartition.

We find that a BHS can halt the dynamical evolution and degree of energy equipartition towards the cluster centre. We also show that the effect grows stronger with the number of stellar-mass black holes in the GC. The signatures introduced by IMBHs depend on how dominant their masses are to the GCs. IMBHs with a mass fraction below 2% of the cluster mass produce a similar dynamical effect to BHS, halting the energy equipartition evolution. However, we find that for IMBHs with a mass fraction larger than 2%, the IMBH can produce an inversion of the observed mass-dependency of the velocity dispersion, where the velocity dispersion grows with mass.

We compare our results from the simulated GCs sample with observations of Galactic GCs and show that real clusters are consistent with our results. In particular, we highlight that some Galactic GCs fall within the anomalous behaviour indicating the presence of BHS or an IMBH and are promising candidates for further dynamical analysis.