Sunday, August 4, 2013

1308.0021 (Oleg Y. Gnedin et al.)

Co-Evolution of Galactic Nuclei and Globular Cluster Systems    [PDF]

Oleg Y. Gnedin, Jeremiah P. Ostriker, Scott Tremaine
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 stellar cluster with an effective radius of several pc, with mass and radius comparable to the typical values in observed nuclear star clusters in late-type and low-mass early-type galaxies. The density contrast associated with the nuclear star cluster is less pronounced in giant elliptical galaxies. Thus disrupted globular clusters may contribute most of the mass of nuclear star clusters (NSCs) in galaxies with stellar mass below 10^{11} Msun. Our results indicate that the NSC mass as a fraction of mass of the galaxy stellar spheroid scales as M_{NSC}/M_{sph} ~ 0.0025 M_{sph,11}^{-0.5}. However, some fraction of the accumulated stellar debris 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. Both the formation time of NSC and the core collapse time are less than 1 Gyr for galaxies more massive than the Milky Way.
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