A search for ultra-compact dwarf galaxies in the Centaurus galaxy cluster ^*,**

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: [ smieske; mhilker; ajordan; mkissler] @eso.org
² Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile e-mail: linfante@astro.puc.cl

Received: 11 April 2007
Accepted: 14 June 2007

Abstract

Aims.Our aim is to extend the investigations of ultra-compact dwarf galaxies (UCD) beyond the well studied Fornax and Virgo clusters.

Methods.We measured spectroscopic redshifts of about 400 compact object candidates with 19.2 < V < 22.4 mag in the central region of the Centaurus galaxy cluster (d = 43 Mpc), using 3 pointings with VIMOS@VLT. The luminosity range of the candidates covers the bright end of the globular cluster (GC) luminosity function and the luminosity regime of UCDs in Fornax and Virgo. Within the area surveyed, our completeness in terms of slit allocation is ≈30%.

Results.We find 27 compact objects with radial velocities consistent with them being members of Centaurus, covering an absolute magnitude range -12.2 < M_V < -10.9 mag. We do not find counterparts to the two very large and bright UCDs in Fornax and Virgo with M_V = -13.5 mag, possibly due to survey incompleteness. The compact objects' distribution in magnitude and space is consistent with that of the GC population. Their kinematics and spatial distribution indicate that they are associated more to the central galaxies than to the overall cluster potential. The compact objects have a mean metallicity consistent with that of the metal-rich globular cluster sub-population. Compact objects with high spectra exhibit solar [ α/Fe] abundances, consistent with typical dwarf elliptical galaxy values and unlike galactic bulge globular clusters. HST based size estimates for a sub-sample of eight compact objects reveal the existence of one very large object with half-light radius r_h around 30 pc, having M_V = -11.6 mag (10⁷ ). This source shows super-solar [ α/Fe] abundances. Seven further sources are only marginally larger than typical GCs with r_h in the range 4 to 10 pc. Those sources exhibit a large scatter in [ α/Fe] abundances.

Conclusions.We consider the largest compact object found to be the only bona fide UCD detected in our study. In order to improve our understanding of UCDs in Centaurus, a significant increase of our survey completeness is necessary.