Ultracompact dwarfs around NGC 3258 in the Antlia cluster

Abstract

We present the first compact stellar systems with luminosities in the range of ultracompact dwarfs (UCDs), discovered in the Antlia galaxy cluster (−10.5 < Mv < −11.6). The magnitude limit between UCDs and globular clusters (CGs) is discussed. By means of imaging from VLT (FORS1), CTIO (MOSAIC) and the Hubble Space Telescope (ACS) archive, 11 UCDs/bright GCs are selected on the basis of photometry and confirmed as Antlia members through radial velocities measured on new Gemini (GMOS) 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 MV ¡­ −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 Reff = 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 Reff and MV is present for UCDs brighter than Mv aprox. −10.5 to −11 mag while no trend is detected for fainter ones, that have an approximately constant Reff. 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.