Star Clusters, Galaxies, and the Fundamental Manifold
  We explore whether global observed properties, specifically half-light radii,
mean surface brightness, and integrated stellar kinematics, suffice to
unambiguously differentiate galaxies from star clusters, which presumably
formed differently and lack dark matter halos. We find that star clusters lie
on the galaxy scaling relationship referred to as the Fundamental Manifold
(FM), on the extension of a sequence of compact galaxies, and so conclude that
there is no simple way to differentiate star clusters from ultra-compact
galaxies. By extending the validity of the FM over a larger range of parameter
space and a wider set of objects, we demonstrate that the physics that
constrains the resulting baryon and dark matter distributions in stellar
systems is more general than previously appreciated. The generality of the FM
implies 1) that the stellar spatial distribution and kinematics of one type of
stellar system do not arise solely from a process particular to that set of
systems, such as violent relaxation for elliptical galaxies, but are instead
the result of an interplay of all processes responsible for the generic
settling of baryons in gravitational potential wells, 2) that the physics of
how baryons settle is independent of whether the system is embedded within a
dark matter halo, and 3) that peculiar initial conditions at formation or
stochastic events during evolution do not ultimately disturb the overall
regularity of baryonic settling. We also utilize the relatively simple nature
of star clusters to relate deviations from the FM to the age of the stellar
population and find that stellar population models systematically and
significantly over predict the mass-to-light ratios of old, metal-rich
clusters.
