4.2 Dwarf Elliptical/Spheroidal Galaxies
Let us first say a few words about the nomenclature. Luminous Elliptical galaxies follow a well defined relation between luminosity and central surface brightness in the sense that more luminous objects have lower central surface brightness (a projection of the fundamental plane), whereas the opposite relation appears to hold for ``diffuse'' early type dwarfs, and late type galaxies (e.g. Binggeli 1994). Thus the kind of galaxy classically termed dwarf elliptical (like Fornax) appears to be physically distinct from non-dwarf elliptical galaxies (like M87), see Binggeli (1994) and Kormendy and Bender (1994). This has created some debate on what to call low mass ellipsoidal galaxies. Ferguson and Binggeli (1994) proposed to call objects with r1/4 profiles (e.g. M32 and giant elliptical galaxies) elliptical (E) and those with more exponential profiles dwarf elliptical (dE). Kormendy and Bender (1994) calls the latter class spheroidal and uses the prefix dwarf to indicate the low luminosity galaxies in each class. Lately, the clear structural distinction between these two classes of objects has begun to be smeared out: Jerjen and Binggeli (1997) show that, as luminosity increases, the luminosity profiles of low mass ellipsoids approach those of giant ellipticals, if the central 0.3 kpc is excluded. In this review we will be primarily interested in the low luminosity low metallicity systems, which we will refer to as dEs, or occasionally dSph (when speaking specifically about the satellites of our Galaxy), which have nearly exponential profiles.
Dwarf elliptical galaxies were long thought to be made up of exclusively old stars, but it has now become evident that many seem to have experienced several star formation episodes, and in many cases quite recently (Grebel 1998). This makes the distinction between dEs and dIs less clear. The SFH in Local Group dEs are mainly explored through their resolved stellar populations, e.g. by means of colour magnitude diagrams. A famous example of a CMD revealing distinct and well separated episodes of star formation is that of the Carina dwarf galaxy (Smecker-Hane et al. 1994).
Masses have been derived for many local dEs using measured velocity dispersions of their stars and they tend to be dark matter dominated (cf. Mateo 1998). Most dEs are very gas poor. In fact, the gas content is sometimes lower than what one would expect from mass loss from the old stellar population alone. The best known example is NGC 147 where neither H I (Young and Lo 1997a) nor CO (Sage et al. 1998) has been found. Thus there might be a mechanism that removes gas, or the gas is in a phase which is not observable. In other dEs, the gas content is in rough agreement with expectations from stellar mass loss (Young and Lo 1997a). An interesting case is the Sculptor dwarf that has no H I within its optical extent, but some well outside in the halo, perhaps moved there by a superwind (Carignan et al. 1998).
Most data on dEs come from the Local Group population, but dEs also exist in large number in galaxy clusters like Virgo, Fornax and Leo. There appear to be very few pure field dEs (i.e. not accompanying a giant galaxy, Binggeli et al. 1990), but their luminosity function is not well known since faint dEs have in general low surface brightness hence may be absent in many surveys (cf. Sect. 6). In the Local Group, new dEs are still beeing discovered (e.g. Armandroff et al. 1998), implying that not even locally do we have a complete picture about the dE population.