Normal Surface Brightness Dwarf Galaxies in the Fornax Cluster

Abstract

There are strong selection effects in favour of selecting galaxies of certain surface brightnesses (Disney & Phillipps 1983; Davies 1990), and against finding galaxies of other surface brightnesses. Both low and high surface brightness objects of a given total luminosity will have faint isophotal magnitudes and/or small isophotal sizes. This means that low surface brightness galaxies, in particular, can generally only be seen if they are quite nearby. Thus if we look in the direction of a cluster, almost all the low surface brightness galaxies we see will be genuine cluster members (Phillipps, these proceedings). ‘Normal’ surface brightness galaxies, on the other hand, are easy to see out to great distances, so they swamp other types of galaxy in any magnitude- or size-limited catalogue (which is why these surface brightnesses are considered ‘normal’, of course). Thus even if we look in a cluster area, apparently faint normal surface brightness objects will be mostly background objects. It is therefore still controversial whether intrinsically low luminosity but normal surface brightness dwarf galaxies exist at all (e.g. Davies et al. 1988; Ferguson & Sandage 1988; Irwin et al. 1990).