The early-type dwarf galaxy population of the Centaurus cluster ^*,**

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: [imisgeld;mhilker]@eso.org
² Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile e-mail: smieske@eso.org

Received: 1 December 2008
Accepted: 21 January 2009

Abstract

Aims. We present a photometric study of the early-type dwarf galaxy population of the Centaurus cluster, aiming at investigating the galaxy luminosity function (LF) and galaxy scaling relations down to the regime of galaxies with  mag.

Methods. On deep VLT/FORS1 V- and I-band images of the central part of the cluster, we identify cluster dwarf-galaxy candidates using both morphological and surface brightness selection criteria. Photometric and structural parameters of the candidates are derived from analysis of their surface brightness profiles. Fundamental scaling relations, such as the colour–magnitude and the magnitude-surface brightness relation, are used to distinguish the cluster from the background.

Results. We find a flat LF with a slope of for  mag, when fitting a power law to the completeness-corrected galaxy number counts. Applying a Schechter function leads to a consistent result of . When plotting the central surface brightness of a Sérsic model vs. the galaxy magnitude, we find a continuous relation for magnitudes  mag, with only the brightest core galaxies deviating from this relation, in agreement with previous studies of other clusters. Within our Centaurus dwarf galaxy sample we identify three very compact objects. We discuss whether they belong to the class of the so-called compact elliptical galaxies (cEs). In a size-luminosity diagram ( vs. M_V) of early-type galaxies from a range of environments, we observe that slowly decreases with decreasing luminosity for  mag and decreases more rapidly at fainter magnitudes. This trend continues to the ultra-faint Local Group dwarf galaxies ( mag).

Conclusions. The continuous central surface brightness vs. absolute magnitude relation and the smooth relation in the size-luminosity diagram over a wide range of magnitudes are consistent with the interpretation of dwarf galaxies and more massive elliptical galaxies being one family of objects with gradually changing structural properties. The most massive core galaxies and the rare cE galaxies are the only exceptions.