Outside of the Magellanic Clouds, dwarf elliptical galaxies (dEs, sometimes also called dwarf spheroidals, or dSph) are the nearest companion galaxies to the Milky Way. The discovery of the tidally distorted Sagittarius dwarf elliptical (Ibata et al. 1994) brought the number of known dE companions for the Milky Way to nine. There may be more still hidden behind the Galaxy's obscuring dust layer. Recent studies have uncovered a host of dE companions of our sister galaxy M31 as well, and a few other more distant 'free-floating' dEs, such as the Cetus galaxy (Whiting et al. 1999) have been found on Schmidt survey plates. Many of the properties of the Local Group dEs have been tabulated by Mateo (1998).
The dEs are deceptively simple stellar systems, with no young stars and apparently kinematically-relaxed stellar populations. Recent high-precision ground-based and HST CCD photometry have demonstrated that this is far from the truth. The dEs display a variety of complex multi-episode star formation histories. For example, Carina shows evidence for several distinct star formation events spread over several Gyr (Smecker-Hane et al. 1994); Sculptor and Fornax appear morphologically similar, but Sculptor appears to formed the bulk of its stars at an earlier time than Fornax (Tolstoy et al. 2001). Only Ursa Minor appears to have something like a simple, monometallic stellar populations based on its color-magnitude diagram (Mighell & Burke 1999, but see below).
These star formation histories are of vital interest to understanding the evolution of the dEs, and they have raised some puzzles. Among these are the question of how, on the one hand, the dEs lost their gas, and how, on the other hand, they retained gas to experience multiple episodes of star formation! Combining the star formation history with the element enrichment history can, in principle, yield the information needed to understand the evolution of dwarf galaxies and their contribution to enrichment of the IGM.