Dissipationless Formation and Evolution of the Milky Way Nuclear Star Cluster

Abstract

In one widely discussed model for the formation of nuclear star clusters (NSCs), massive globular clusters spiral into the center of a galaxy and merge to form the nucleus. It is now known that at least some NSCs coexist with supermassive black holes (SBHs); this is the case, for instance, in the Milky Way. In this paper, we investigate how the presence of a SMBH at the center of the Milky Way impacts the merger hypothesis for the formation of its NSC. Starting from a model consisting of a low-density nuclear stellar disk and the SMBH, we use direct N -body simulations to follow the successive inspiral and merger of globular clusters. The clusters are started on circular orbits of radius 20 pc, and their initial masses and radii are set up in such a way as to be consistent with the galactic tidal field at that radius. These clusters, decayed orbitally in the central region due to their large mass, were followed in their inspiral events; as a result, the total accumulated mass by ≈ 10 clusters is about 1.5 × 10 7 M ⊙ . Each cluster is disrupted by the SMBH at a distance of roughly one parsec. The density profile that results after the final inspiral event is characterized by a core of roughly this radius, and an envelope with density that falls off ρ ∼ r −2 . These properties are similar to those of the Milky Way NSC, with the exception of the core size, which in the Milky Way is somewhat smaller. But by continuing the evolution of the model after the final inspiral event, we find that the core shrinks substantially via gravitational encounters in a time (when scaled to the Milky Way) of 10 Gyr as the stellar distribution evolves toward a Bahcall-Wolf cusp. We also show that the luminosity function of the Milky Way NSC is consistent with the hypothesis that 1/2 of the mass comes from old (∼ 10 Gyr) stars, brought in by globular clusters, with the other half due to continuous star formation. We conclude that a model in which a large fraction of the mass of the Milky Way is due to infalling globular clusters is consistent with existing observational constraints.

The old nuclear star cluster in the Milky Way: dynamics, mass, statistical parallax, and black hole mass

“…Subsequently, relaxation would lead to shrinking of the core by a factor of ∼ 2 in 10 Gyr towards a value similar to that observed (Merritt 2010). In the simulations of Antonini et al (2012), the final relaxed model has an inner slope of γ = 0.45, not far from our models (note that in flattened semi-isotropic models the minimum allowed slope for the density is also 0.5 (Qian et al 1995)). Their cluster also evolved towards a more spherical shape, however, starting from a configuration with much less rotation and flattening than we inferred here for the present Milky Way NSC.…”

“…In the latter model, if the black hole is already present, the NSC density and rotation after completion of the merging phase reflects the distribution of disrupted material in the potential of the black-hole (e.g. Antonini et al 2012). Subsequently, relaxation would lead to shrinking of the core by a factor of ∼ 2 in 10 Gyr towards a value similar to that observed (Merritt 2010).…”

The formation of stellar nuclear discs in bar-induced gas inflows

“…Our model supports the view that gas can be fed to such small radii to fuel ongoing episodic star formation (Pfuhl et al 2011). Though it is clear that the merger of globular clusters at the centre is a plausible process for growing nuclear star clusters (Tremaine, Ostriker & Spitzer 1975;Capuzzo-Dolcetta 1993;Capuzzo-Dolcetta & Miocchi 2008;Antonini et al 2012;Antonini 2013;Gnedin, Ostriker & Tremaine 2014), helping explain some kinematic kinematic anomalies, (Hartmann et al 2011;De Lorenzi et al 2013), it is likely that insitu star formation plays a more significant role.…”

The S-star cluster at the center of the Milky Way

“…Dynamically unrelaxed models imply a much lower density near Sgr A* and an uncertain fraction of stellar-mass black holes (Merritt 2010;Antonini et al 2012). The number of perturbers is so small in these models that their effect on the orbital elements of S2 would be undetectable for the foreseeable future, barring a lucky close encounter with S2.…”