Ultra-Compact Dwarfs around NGC 3258 in Antlia (1305.1896v1)

Abstract: We present the first compact stellar systems with luminosities in the range of ultra-compact dwarfs (UCDs), discovered in the Antlia galaxy cluster (-10.5 < M_V < -11.6). The magnitude limit between UCDs and globular clusters (CGs) is discussed. By means of imaging from VLT (FORS1), CTIO (MOSAIC), and the HST (ACS) archive, eleven UCDs/bright GCs are selected on the basis of photometry and confirmed as Antlia members through radial velocities measured on new GEMINI (GMOS-S) spectra. In addition, nine UCD candidates are identified taking into account properties derived from their surface brightness profiles. All of them, members and candidates, are located in the proximity of NGC\,3258, one of the two brightest elliptical galaxies in the cluster core. Antlia UCDs in this sample present absolute magnitudes fainter than M_V ~ -11.6 mag and most of them have colours within the blue GC range, falling only two within the red GC range. Effective radii measured for the ones lying on the ACS field are in the range R_eff = 3 - 11 pc and are similar to equivalent objects in other clusters, obtained from the literature. The UCD sample shares the same behaviour on the size-luminosity plane: a linear relation between R_eff and M_V is present for UCDs brighter than M_V ~ -10.5 - -11 mag while no trend is detected for fainter ones, that have an approximately constant R_eff. The projected spatial distribution of UCDs, GCs and X-ray emission points to an ongoing merger between two Antlia groups, dominated by NGC 3258 and NGC 3268. Nuclei of dwarf elliptical galaxies and blue UCDs share the same locus on the colour-magnitude diagram, supporting the hypothesis that some blue UCDs may be remnants of stripped nucleated dwarfs.