Origin and growth of nuclear star clusters around massive black holes

TL;DR

This paper explores the origin of nuclear star clusters (NSCs) around massive black holes, proposing that NSCs form from the merger of stellar clusters influenced by dynamical processes, with implications for their presence in different galaxy types.

Contribution

It demonstrates that NSCs likely originate from cluster mergers rather than in-situ formation and explains their absence in galaxies with very massive black holes.

Findings

NSC mass relates to host galaxy velocity dispersion consistent with merger models.

Dynamical friction causes stellar clusters to migrate and merge into NSCs over 10 Gyr.

Presence of a massive black hole disrupts clusters, reducing NSC density.

Abstract

The centers of stellar spheroids are often marked by the presence of nucleated central regions, called nuclear star clusters (NSCs). The origin of NSCs is still unclear. Here we investigate the possibility that NSCs originate from the migration and merger of stellar clusters at the center of galaxies where a massive black hole (MBH) may sit. We show that the observed relation between NSC masses and the velocity dispersion of their host spheroids cannot be reconciled with a purely in-situ formation scenario. On the other hand, the observed relation appears to be in agreement with the predictions of the cluster merger model which also reproduces the observed relation between the size of NSCs and their total luminosity. We evolve through dynamical friction a population of stellar clusters in a model of a galactic bulge taking into account dynamical dissolution, starting from a power-law cluster initial mass function and a total mass in stellar clusters consistent with the cluster formation efficiency of the Milky Way (MW). The most massive clusters reach the center of the galaxy and merge to form a compact nucleus; after 10 Gyr, the resulting NSC has properties that are consistent with the observed distribution of stars in the MW NSC. When a MBH is included at the center of a galaxy, globular clusters are tidally disrupted during inspiral, resulting in NSCs with lower densities than those of NSCs forming in galaxies with no MBHs. We suggest this as a possible explanation for the lack of NSCs in galaxies containing MBHs more massive than ~10^8M_Sun. Finally, we investigate the orbital evolution of globular clusters in giant elliptical galaxies which are believed to always host a MBH at their center rather than a NSC. In these systems an additional mechanism can prevent a NSC from forming: the time for globular clusters to reach the center of the galaxy is much longer than the Hubble time.