Co-Evolution of Galactic Nuclei and Globular Cluster Systems
  We revisit the hypothesis that dense galactic nuclei are formed from
inspiraling globular clusters. Recent advances in understanding of the
continuous formation of globular clusters over cosmic time and the concurrent
evolution of the galaxy stellar distribution allow us to construct a simple
model that matches the observed spatial and mass distributions of clusters in
the Galaxy and the giant elliptical galaxy M87. In order to compare with
observations, we model the effects of dynamical friction and dynamical
evolution, including stellar mass loss, tidal stripping of stars, and tidal
disruption of clusters by the growing galactic nucleus. We find that
inspiraling globular clusters form a dense central structure, with mass and
radius comparable to the typical values in observed nuclear star clusters
(NSCs) in late-type and low-mass early-type galaxies. The density contrast
associated with the NSC is less pronounced in giant elliptical galaxies. Our
results indicate that the NSC mass as a fraction of mass of the galaxy stellar
spheroid scales as M_NSC/M_* = 0.0025 M_{*,11}^{-0.5}. Thus disrupted globular
clusters could contribute most of the mass of NSCs in galaxies with stellar
mass below 10^{11} Msun. The inner part of the accumulated cluster may seed the
growth of a central black hole via stellar dynamical core collapse, thereby
relieving the problem of how to form luminous quasars at high redshift. The
seed black hole may reach about 10^5 Msun within 1 Gyr of the beginning of
globular cluster formation.
