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

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: smieske@eso.org
² Argelander Institut für Astronomie, Abteilung Sternwarte, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: mhilker@astro.uni-bonn.de
³ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile e-mail: linfante@astro.puc.cl
⁴ Instituto de Astronomia, Geofísica, e Ciências Atmosféricas, Departamento de Astronomia, Universidade de São Paulo, Rua do Matáo 1226, Cidade Universitãria, 05508-900 São Paulo, SP, Brazil e-mail: oliveira@astro.iag.usp.br

Received: 25 August 2006
Accepted: 17 October 2006

Abstract

Aims.We analyse the photometric properties of the early-type Fornax cluster dwarf-galaxy population ( mag), based on a wide-field imaging study of the central cluster area in V and I bandpasses. We used the instrument/telescope combination IMACS/Magellan at Las Campanas Observatory, providing much larger light-collecting area and better image resolution than previous wide-field imaging surveys.

Methods.We created a fiducial sample of Fornax cluster dwarf ellipticals (dEs) in the following three steps. (1) To verify cluster membership, we measured I-band surface brightness fluctuations (SBF) distances to candidate dEs known from previous surveys. (2) We re-assessed morphological classifications for those candidate dEs that are too faint for SBF detection. (3) We searched for new candidate dEs in the size-luminosity regime close to the resolution limit of previous surveys.

Results.(1) We confirm cluster membership for 28 candidate dEs in the range  mag by means of SBF measurement. We find no SBF background galaxy. (2) Of 51 other candidate dEs in the range  mag, 2/3 are confirmed as probable cluster members by morphological re-assessment, while 1/3 are re-classified as probable background objects. (3) We find 12 new dE candidates in the range  mag, two of which are directly confirmed via SBF measurement. The resulting fiducial dE sample follows a well-defined surface brightness–magnitude relation, showing that Fornax dEs are about 40% larger than Local Group dEs. The sample also defines a colour–magnitude relation that appears slightly shallower than that of Local Group dEs. The early-type dwarf galaxy luminosity function in Fornax has a very flat faint end slope . We discuss these findings in the context of structure-formation theories.

Conclusions.The SBF method is a very powerful tool to help constrain the faint end of the galaxy luminosity function in nearby galaxy clusters. For the Fornax cluster, morphological cluster memberships – if performed at sufficient resolution – are very reliable.