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 UCDs, confirms the existence of two distinct formation channels for these compact stellar systems. We find that the UCDs we have discovered around the group elliptical NGC 3923 (and most UCDs in general) have properties consistent with their being the most luminous members of the host galaxy's globular cluster (GC) system. As with GCs they are therefore likely to be the product of the most violent epochs of galaxy formation. We describe UCDs of this type as giant GCs (GGCs). In contrast, the UCD we have found associated with the isolated S0 NGC 4546 is clearly the result of the stripping of a nucleated companion galaxy. The young age (similar to 3.4 Gyr) of the UCD, the lack of a correspondingly young GC population, the apparently short dynamical friction decay time-scale (similar to 0.5 Gyr) of the UCD and the presence of a counter-rotating gas disc in the host galaxy (corotating with respect to the UCD) together suggest that this UCD is the liberated nucleus remaining after the recent stripping of a companion by NGC 4546. We infer that the presence of UCDs of either category (GGCs formed in major star-forming events, or stripped nuclei formed in minor mergers) can provide a useful probe of the assembly history of the host galaxy. 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 2 x 10(6) M-circle dot (M-V similar to -10), UCDs emerge together with a mass-size relation and a likely mass-metallicity relation (the 'blue tilt'). In the mass range up to 7 x 10(7) M-circle dot (M-V similar to -13) UCDs comprise a composite population of GGCs and stripped nuclei. Interestingly, dwarf nuclei have similar colours to blue GCs and UCDs across the scaling onset mass, smoothly extending the blue tilt, while nuclei of more massive galaxies and a prominent minority of UCDs extend the red locus of GCs. Above 7 x 10(7) M-circle dot, UCDs must be almost exclusively stripped nuclei, as no sufficiently rich GC systems exist to populate such an extreme of the GCLF.