2011ApJ...737L..13M

Eleven ultra-compact dwarf (UCD) and 39 extended star cluster candidates are found to be associated with the galaxy NGC 1132. This giant elliptical galaxy is the remnant of a fossil group. UCD and extended star cluster candidates are identified through the analysis of their structural parameters, colors, spatial distribution, and luminosity using deep Hubble Space Telescope observations in two filters: the F475W (Sloan g) and F850LP (Sloan z). The median effective radius of these UCDs is r_h= 13.0 pc. Two types of UCDs are identified in the vicinity of NGC 1132: one type shares the same color and luminosity as the brightest globular clusters and traces the onset of the mass-size relation. The second kind of UCD is represented by the brightest UCD candidate, an M32-type object, with an effective radius of r_h= 77.1 pc, located at ∼6.6 kpc from the nucleus of NGC 1132. This UCD candidate is likely the remaining nucleus of a minor merger with the host galaxy. With the exception of a particularly blue UCD candidate, UCDs are found to extend the mass-metallicity relation found in globular clusters to higher luminosities. The results of this work support the growing body of evidence showing that UCDs are not circumscribed to galaxy clusters as previously thought. UCDs are likely to be a common occurrence in all environments. The milder tidal field of a fossil group, when compared to a galaxy cluster, allows UCDs and extended star clusters to survive up to the present time at small galactocentric distances.