A search for massive ultra-compact dwarf galaxies in the Centaurus galaxy cluster

Abstract

Context. In a previous paper, we initiated a search for ultra-compact dwarf galaxies (UCDs) in the Centaurus galaxy cluster, resulting in the discovery of 27 compact objects with -12.2 < Mv < -10.9mag. Our overall survey completeness was 15-20% within 120 kpc projected clustercentric distance.Aims. To constrain the luminosity distribution of the brightest UCDs in Centaurus in a more accurate way, we continue our search by improving substantially our survey completeness specifically for brightnesses of M v < -12 mag (V0 < 21.3 mag).Methods. Using VIMOS at the VLT, we obtained low resolution spectra of 400 compact objects with 19.3 < 21.3 mag (-14 < Mv < -12 mag at the Centaurus distance) in the central 25' of the Centaurus cluster, which corresponds to a projected radius of ~150 kpc. Our survey had complete area coverage within a radius of ∼150 kpc.Results. For 94% of the sources included in the masks, we measure successfully a redshift. Because of incompleteness in the slit assignment, our final completeness in the area surveyed is 52%. Among our targets, we find three new UCDs in the magnitude range -12.2 < Mv < -12 mag, hence at the faint limit of our survey. For one of these targets HST WFPC2 imaging is available, yielding a size estimate of ≲ 8-9 pc. At 95% confidence, we can reject the hypothesis that in the area surveyed there are more than 2 massive UCDs with Mv < -12.2 mag and reff ≲70pc. Hence our survey confirms the extreme rarity of massive UCDs. We find that the radial distributions of Centaurus and Fornax UCDs with respect to their host clusters' centres agree within the 2σ level. © 2009 ESO.