Violent relaxation in hierarchical stellar systems: pathways toward massive black hole formation in the early Universe

Abstract

Massive stellar systems have been detected in the high-redshift Universe via the James Webb Space Telescope using gravitational lensing techniques. Such stellar systems may evolve on short dynamical timescales, despite a formally large 2-body relaxation time. If the stellar system consists of a large number of gravitationally bound clusters, a 2-body relaxation time can be defined for the star clusters in analogy to the 2-body relaxation time for stars, implying a faster evolution determined by the number of stars per cluster. At their formation, clusters will go through a process of "cold collapse" as part of their early, non-collisional, evolution. This phase enhances the mass segregation between the more massive and less massive population of stars. We derive a criteria based on the mass of the two components that determines under which conditions the massive component may even decouple from the cluster itself. Mergers of clusters and multiple mergers on average tend to produce systems with larger cluster radii. For the formation of massive black holes via dense stellar systems, the most promising pathway consists in the cold collapse of a dense massive cluster, after few mergers (or possibly none). In such a cluster with a sufficient number of massive stars, the massive population of stars will dynamically decouple in the central region and go through core collapse, leading to the formation of a central massive object. In the case of multiple mergers, massive but more spatially extended stellar systems will form.