The early - type dwarf galaxy population of the Fornax cluster

Aims. We analyse the photometric properties of the early-type Fornax cluster dwarf galaxy population (M V > −17 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 create 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 reassessed morphological classifications for those candidate dEs that are too faint for SBF detection; and (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 −16.6 < M V < −10.1 mag by means of SBF measurement. We find no SBF background galaxy. (2) Of 51 further candidate dEs in the range −13.2 < M V < −8.6 mag, 2/3 are confirmed as probable cluster members by morphological re-assessment, while 1/3 are reclassified as probable background objects. (3) We find 12 new dE candidates in the range −12.3 < M V < −8.8 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 α ≃ −1.1 ± 0.1. 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.