Fabio Antonini, Roberto Capuzzo-Dolcetta, Alessandra Mastrobuono-Battisti, David Merritt
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 (MW). In this paper, we investigate how the presence of a SMBH at the
center of the MW 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 N-body simulations to follow the successive inspiral and merger of
(12) 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. The total accumulated
mass is about 1.5x10^7 Solar masses. 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 rho \sim r^-2. These properties are
similar to those of the MW NSC, with the exception of the core size, which in
the MW is a little 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 MW) of 10 Gyr as the
stellar distribution evolves toward a Bahcall-Wolf cusp. We also show that the
luminosity function of the MW NSC is consistent with the hypothesis that a
large fraction of the mass comes from (~10Gyr) old stars, brought in by
globular clusters. We conclude that a model in which a large fraction of the
mass of the MW NSC arose from infalling globular clusters is consistent with
existing observational constraints.
View original:
http://arxiv.org/abs/1110.5937
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