An Extended Star Formation History in an Ultra Compact Dwarf
  There has been significant controversy over the mechanisms responsible for
forming compact stellar systems like ultra compact dwarfs (UCDs), with
suggestions that UCDs are simply the high mass extension of the globular
cluster (GC) population, or alternatively, the liberated nuclei of galaxies
tidally stripped by larger companions. Definitive examples of UCDs formed by
either route have been difficult to find, with only a handful of persuasive
examples of stripped-nucleus type UCDs being known. In this paper we present
very deep Gemini/GMOS spectroscopic observations of the suspected stripped
nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (< 0.7"). With these
data we examine the spatially resolved kinematics and star formation history of
this unusual object. We find no evidence of a rise in the central velocity
dispersion of the UCD, suggesting that this UCD lacks a massive central black
hole like those found in some other compact stellar systems, a conclusion
confirmed by detailed dynamical modelling. Finally we are able to use our
extremely high signal to noise spectrum to detect a temporally extended star
formation history for this UCD. We find that the UCD was forming stars since
the earliest epochs until at least 1-2 Gyr ago. Taken together these
observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated
dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1-2 Gyr.
