1.2 A brief history

Studies of dwarf elliptical galaxies began with Shapley's discovery of the Fornax and Sculptor dE companions to the Milky Way (Shapley 1938a). At that time it was suspected that more luminous examples of such systems might exist in the Virgo cluster (Shapley 1938b), and similar companions to M31 were soon identified. Baade (1944a; 1944b) and Shapley were able to resolve the brightest stars in Local Group dwarf spheroidals and identify these systems as ``population II'' objects, lacking the luminous OB stars present in the galactic disk. While these systems were similar to globular clusters and elliptical galaxies in their stellar populations and smooth structure, the low surface density of stars prompted Baade (1944b) to remark that ``a marked change in the internal structure of E-type nebulae takes place as we reach the systems of the lowest luminosity. The strong concentration toward the center and the central nucleus disappear gradually until we encounter such limiting forms as the Sculptor and Fornax Systems''. From the numbers of Local Group dwarfs known in 1944, Baade also noted that ``there seems little doubt that the symmetrical form of the luminosity function hitherto adopted has to be replaced with a skew distribution.'' Baade's conclusions about the structure and luminosity function of low-luminosity elliptical galaxies are echoed in modern studies of the dE populations in clusters of galaxies.

Identification of more Local Group dwarfs continued with the advent of the Palomar Sky Survey (Harrington and Wilson 1950; Wilson 1955). Searches for low-surface-brightness objects yielded similar objects outside the Local Group (Van den Bergh 1959; 1972), often concentrated around a nearby giant galaxy (Holmberg 1950; 1969). In parallel with the early identifications of nearby dwarf spheroidals and dwarf irregulars, evidence accumulated for a significant population of low-surface-brightness dwarf galaxies in nearby clusters (Shapley 1943; Reaves 1956; Hodge 1959; Hodge 1960; Hodge et al. 1965). Their spatial distribution relative to the surrounding bright galaxies immediately implicated these faint, diffuse galaxies as cluster members. However, because of their low surface-brightnesses, the systematic properties of cluster dwarfs remained largely unexplored until large-scale photographic surveys of nearby clusters allowed a detailed examination of their spatial distribution, luminosity function, and structural scaling relations (e.g. Binggeli et al. 1985; Caldwell 1987; Ferguson and Sandage 1988), and spectroscopy and multi-color observations began to reveal the properties of their stellar populations (Bothun and Caldwell 1984; Bothun et al. 1985).

The inventory of Milky Way satellites now appears virtually complete, at least out of the plane and down to luminosities M_B -9 (Irwin 1994). Because of the difficulty of obtaining redshifts, catalogues of dE galaxies in other environments are less complete and more prone to contamination. Companions of galaxies within ~ 5 Mpc have been identified in various studies (see Table 1, but the physical association is in many cases uncertain. The problem of contamination is less severe in the richest local environments (the Virgo and Fornax clusters), as the contrast of these clusters with the surrounding background is much higher and the clusters are close enough that dE galaxies can be readily distinguished from background galaxies. The principal difficulty in these surveys is incompleteness, both at low surface brightnesses due to the sensitivity of photographic plates (Impey et al. 1988) and at the high surface brightnesses due to confusion with background objects. At velocities more than about twice that of Virgo, the task of identifying dE galaxies becomes much more difficult, and contamination by background galaxies is again a concern. By distances of ~ 5000 km s^-1, the separation of cluster and background is best done statistically by comparison with blank fields. With the exception of Coma (Thompson and Gregory 1993), the dE population in rich clusters is essentially unknown. Populations of dE galaxies have been identified in a few loose groups with v < 2700 km s^-1 (Ferguson and Sandage 1991), and as companions of bright elliptical galaxies (Vader and Sandage 1991). Only a few examples of isolated dE galaxies are known, but the selection biases against finding such galaxies are severe. The clustering properties of dE galaxies are discussed in Sect. 6.