The Ubiquity and Dual Nature of Ultra Compact Dwarfs
  We present the discovery of several Ultra Compact Dwarfs (UCDs) located in
field/group environments. Examination of these objects, plus literature
objects, confirms the existence of two distinct formation channels for UCDs. We
find that the UCDs we have discovered around the group elliptical NGC3923 (and
UCDs generally) have properties consistent with their being the most luminous
members of the host galaxy's globular cluster (GC) system. We describe UCDs of
this type as giant GCs (GGCs). In contrast, the UCD we have found associated
with the isolated S0 NGC4546 is clearly the result of the stripping of a
nucleated companion galaxy. The young age (~3.4 Gyr) of the UCD, the lack of a
correspondingly young GC population, the apparently short dynamical friction
decay timescale (~0.5 Gyr) of the UCD, and the presence of a counterrotating
gas disc in the host galaxy (co-rotating with the UCD) together suggest that
this UCD is the liberated nucleus remaining after the recent stripping of a
companion by NGC4546. We suggest a general scheme that unifies the formation of
GCs, UCDs, and galaxy nuclei. In this picture "normal" GCs are a composite
population, composed of GCs formed in situ, GCs acquired from accreted
galaxies, and a population of lower mass stripped dwarf nuclei masquerading as
GCs. Above a "scaling onset mass" of 2x10^6 Msun (Mv ~ -10), UCDs emerge
together with a mass-size relation and a likely mass-metallicity relation (the
"blue tilt"). In the mass range up to 7x10^7 Msun (Mv ~ -13) UCDs comprise a
composite population of GGCs and stripped nuclei. Above 7x10^7 Msun, UCDs must
be almost exclusively stripped nuclei, as no sufficiently rich GC systems exist
to populate such an extreme of the GCLF.
