Ultra-Compact Dwarf Candidates Near the Lensing Galaxy in Abell S0740

Abstract

We analyze three-band HST/ACS imaging data of the giant elliptical galaxy ESO325-G004. This is the nearest known strongly lensing galaxy, and it resides in the center of the poor cluster Abell S0740 at redshift z=0.034. Based on magnitude, color, and size selection criteria, we identify a sample of 15 ultra-compact dwarf (UCD) galaxy candidates within the ACS field. This is comparable to the numbers of UCDs found within similar regions in more nearby clusters (Virgo, Fornax, Hydra). We estimate circular half-light radii R_e,c from 2-D Sersic and King model fits and apply an upper cutoff of 100pc for our UCD selection. The selected galaxies have typical Sersic indices $n\approx1.5$, while larger sources with R_e,c > 100pc are more nearly exponential, perhaps indicating that the latter are dominated by background disk galaxies. Many of the UCD candidates are surrounded by a faint "fuzz" of halo light, which may be the remnants of stripped material, and there is some evidence for intrinsic flattening of the UCDs themselves. An apparent separation in size between the most compact UCDs with R_e,c < 17pc and larger ones with R_e,c > 40pc may hint at different formation mechanisms. We do not find any M32 analogues in this field. The colors of the UCD candidates span the range from blue to red globular clusters, although the brightest ones are predominantly red. The UCD candidates follow the flattened, elliptical distribution of the globular clusters, which in turn follow the galaxy halo light, suggesting a common evolution for these three components. Follow-up spectroscopy can determine which candidates are truly members of Abell S0740 and how similar they are in distribution to the globulars.