Next Contents Previous

1.1 What is a dE Galaxy?

Low-luminosity elliptical galaxies are distinguished from late-type galaxies (spirals and irregulars) by their smooth surface-brightness profiles. Below luminosities of MB approx -18 the smooth-profile galaxies divide into two classes: compact galaxies with high central surface brightnesses (exemplified by M32), and diffuse galaxies with low central surface brightnesses (exemplified by the Local Group dwarf spheroidals). The terms ``dwarf elliptical'' (dE) and ``dwarf spheroidal'' (dSph) have been used most often to describe smooth, low surface brightness (LSB) galaxies. However, the lack of a universally accepted definition has led to some confusion over whether these terms refer to the same thing, and in particular whether the dE class includes galaxies like M32. In the discussions that follow, we adopt the classification scheme set out in the extensive Virgo cluster dwarf atlas of Sandage and Binggeli (1984). In this scheme the term dE encompasses both local dSph galaxies and similar-looking galaxies beyond the Local Group. Faint ellipticals with profiles that are more nearly r1/4-law are referred to simply as ``ellipticals'' (E), or sometimes ``compact ellipticals,'' but never dE's. A different name for ``dwarf elliptical'' frequently encountered in the literature is ``spheroidal''. The pros and cons of either name convention are discussed in Binggeli (1994b) and Kormendy and Bender (1994).

More detailed issues of morphology, including the quantitative distinctions between the different classes and the existence of intermediate types, are discussed in Sect. 2.1.

All distance-dependent quantities discussed in this paper are based on a long distance scale (H0 approx 50 km s-1 Mpc-1), although in most cases the distances are not estimated from radial velocities.