ULTRA-compact DWARF candidates near the lensing galaxy in abell S0740

Abstract

We analyze three-band imaging data of the giant elliptical galaxy ESO325-G004 from the Hubble Space Telescope Advanced Camera for Surveys (ACS). 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 number of UCDs found within similar regions in more nearby clusters (Virgo, Fornax, Hydra). We estimate circular half-light radii Re,c from two-dimensional Sérsic and King model fits and apply an upper cutoff of 100 pc for our UCD selection. The selected galaxies have typical Sérsic indices n 1.5, while larger sources with Re,c > 100 pc 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 Re,c < 17 pc and larger ones with Re,c > 40 pc 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 (GCs), although the brightest ones are predominantly red. The UCD candidates follow the flattened, elliptical distribution of the GCs, which in turn follow the galaxy halo light, suggesting a common evolution for these three components. Planned follow-up spectroscopy can determine which candidates are truly members of Abell S0740 and how similar they are in distribution to the GCs. © 2008. The American Astronomical Society. All rights reserved.