Abstract
Nuclear star clusters (NSCs) are dense stellar clusters observed in galactic nuclei, typically hosting a central massive black hole. Here we study the possible formation and evolution of NSCs through the inspiral of multiple star clusters hosting intermediate mass black holes (IMBHs). Using an N-body code we examine the dynamics of the IMBHs and their effects on the NSC. We find that IMBHs inspiral to the core of the newly formed NSC and segregate there. Although the IMBHs scatter each other and the stars, none of them is ejected from the NSC. The IMBHs are excited to high eccentricities and their radial density profile develops a steep power-law cusp. The stars also develop a power-law cusp (instead of the central core that forms in their absence), but with a shallower slope. The relaxation rate of the NSC is accelerated due to the presence of IMBHs, which act as massive-perturbers. This in turn fills the loss-cone and boosts the tidal disruption rate of stars both by the MBH and the IMBHs to a value excluded by rate estimates based on current observations. Rate estimates of tidal disruptions can therefore provide a cumulative constraint on the existence of IMBHs in NSCs.
Highlights
Nuclear stellar clusters (NSCs) are dense and compact stellar systems observed in galactic nuclei, many of which hosting a massive black hole (MBH) at their center (Boker 2010)
In the simulations run without the central intermediate mass black holes (IMBHs) (Antonini et al 2012), the infalling clusters (ICs) are disrupted at ∼2 pc, and their stars develop a central flat core
In comparison, following the disruption of the IMBH-hosting ICs, the IMBHs keep inspiraling to the center and lead to the fast relaxation of the NSC stellar population, giving rise to a central cusp, with a somewhat shallower slope than a Bahcall & Wolf (1976) profile, likely due to the strong mass segregation induced by the IMBHs
Summary
Nuclear stellar clusters (NSCs) are dense and compact stellar systems observed in galactic nuclei, many of which hosting a massive black hole (MBH) at their center (Boker 2010). Two main hypotheses have been suggested to explain the origin of NSCs: (1) the in-situ formation, where molecular gas coming from more external regions is channeled to the GC leading to an episodic star formation epoch (Milosavljevic 2004); and (2) the cluster-infall/merger scenario, where dense massive stellar clusters decay toward the center of their host galaxy due to dynamical friction, merge and form the NSC. These two scenarios are not mutually exclusive, and both can contribute to the formation of NSCs. Here we focus only the cluster-infall scenario
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