The Relationships Among Compact Stellar Systems: A Fresh View of Ultra
  Compact Dwarfs
  We use a combined imaging and spectroscopic survey of the nearby central
cluster galaxy, M87, to assemble a sample of 34 confirmed ultra compact dwarfs
(UCDs) with half-light radii of >~ 10 pc measured from Hubble Space Telescope
images. This doubles the existing sample in M87, making it the largest such
sample for any galaxy, while extending the detection of UCDs to unprecedentedly
low luminosities (MV = -9). With this expanded sample, we find no correlation
between size and luminosity, in contrast to previous suggestions, and no
general correlation between size and galactocentric distance. We explore the
relationships between UCDs, less luminous extended clusters (including faint
fuzzies), globular clusters (GCs), as well as early-type galaxies and their
nuclei, assembling an extensive new catalog of sizes and luminosities for
stellar systems. Most of the M87 UCDs follow a tight color-magnitude relation,
offset from the metal-poor GCs. This, along with kinematical differences,
demonstrates that most UCDs are a distinct population from normal GCs, and not
simply a continuation to larger sizes and higher luminosities. The UCD
color-magnitude trend couples closely with that for Virgo dwarf elliptical
nuclei. We conclude that the M87 UCDs are predominantly stripped nuclei. The
brightest and reddest UCDs may be the remnant nuclei of more massive galaxies
while a subset of the faintest UCDs may be tidally limited and related to more
compact star clusters. In the broader context of galaxy assembly, blue UCDs may
trace halo build-up by accretion of low-mass satellites, while red UCDs may be
markers of metal-rich bulge formation in larger galaxies.
