A search for massive UCDs in the Centaurus Galaxy Cluster
  We recently initiated a search for ultra-compact dwarf galaxies (UCDs) in the
Centaurus galaxy cluster (Mieske et al. 2007), resulting in the discovery of 27
compact objects with -12.2<M_V<-10.9 mag. Our overall survey completeness was
15-20% within 120 kpc projected clustercentric distance. In order to better
constrain the luminosity distribution of the brightest UCDs in Centaurus, we
continue our search by substantially improving our survey completeness
specifically in the regime M_V<-12 mag (V_0<21.3 mag). Using VIMOS at the VLT,
we obtain low-resolution spectra of 400 compact objects with 19.3<V_0<21.3 mag
(-14<M_V<-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 yields
complete area coverage within ~120 kpc. For 94% of the sources included in the
masks we successfully measure a redshift. Due to 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<M_V<-12 mag, hence
at the faint limit of our survey. One of them is covered by archival HST WFPC2
imaging, yielding a size estimate of r_h <= 8-9 pc. At 95% confidence we can
reject the hypothesis that in the area surveyed there are more than 2 massive
UCDs with M_V<-12.2 mag and r_eff <=70 pc. Our survey hence confirms the
extreme rareness of massive UCDs. We find that the radial distributions of
Centaurus and Fornax UCDs with respect to their host clusters' centers agree
within the 2 sigma level.
