The early-type dwarf galaxy population of the Hydra I cluster
Abstract
We analyse the properties of the early-type dwarf galaxy population in the Hydra I cluster. We investigate the galaxy luminosity function (LF), the colour-magnitude relation (CMR), and the magnitude-surface brightness relation down to MV~-10 mag. Another goal of this study is to find candidates for ultra-compact dwarf galaxies (UCDs). Two spectroscopic surveys performed with Magellan I/LDSS2 and VLT/VIMOS, as well as deep VLT/FORS1 images in V and I bands were examined. We identify cluster members by radial velocity measurements and select other cluster galaxy candidates by their morphology. One possible UCD candidate with MV=-13.26 mag is found. Our sample of 100 morphologically selected dwarf galaxies defines a CMR that extends the CMR of the giant cluster galaxies to the magnitude limit of our survey (MV~-10 mag). It matches the relations found for the Local Group and the Fornax cluster dwarfs almost perfectly. The Hydra I dwarfs also follow a magnitude-surface brightness relation similar to that of the LG dwarfs. Moreover, we observe a continuous relation for dwarf galaxies and giant early-type galaxies when plotting the central surface brightness μ0 of a S\'ersic model vs. the galaxy magnitude. The effective radius is found to be largely independent of the luminosity for MV>-18 mag, being Re~0.8 kpc. We derive a very flat faint-end slope of the LF (α = -1.13 0.04) from fitting a Schechter function, whereas fitting a power law for MV>-14 mag gives α = -1.40 0.18. Our findings suggest that early-type dwarf and giant galaxies are the same class of objects. The similarity of the dwarf galaxy scaling relations to other environments implies that internal processes could be more important for their global photometric properties than external influences. (abridged)