The ubiquity and dual nature of ultra-compact dwarfs

Abstract

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.