Dwarf elliptical galaxies, or dE's, are elliptical galaxies that are much smaller than others, classified as dE. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies. While the surface brightness profiles of giant elliptical galaxies are well described by de Vaucouleur's law, dE's have exponentially declining surface brightness profiles. Dwarf elliptical galaxies are, therefore, much more diffuse than giant elliptical galaxies. Dwarf elliptical galaxies have blue absolute magnitudes within the range -18 mag < M < -14 mag. Still fainter elliptical-like galaxies are called dwarf spheroidal galaxies. Dwarf ellipticals may be primordial objects. Within the currently favoured cosmological Lambda-CDM model, small objects (consisting of dark matter and gas) are the first to form. Because of their mutual gravitational attraction, some of these will coalesce and merge, forming more massive objects. Further mergers lead to ever more massive objects. This road to the present-day galaxies has been called "hierarchical merging". If this theory is correct, dwarf galaxies may be the building blocks of today's giant galaxies. Alternatively, Ben Moore of the University of Zurich, Switzerland, proposed in 1996 that dE's could also be the remnants of low-mass spiral galaxies that through the action of repeated gravitational interactions with giant galaxies within a cluster obtained a much rounder shape. This process of changing a galaxy's morphology by interactions has been called "galaxy harassment". Evidence for this latter theory has been found, in the form of stellar disks and spiral arms (all that is left of the original stellar disk of the transformed spiral galaxy) embedded within dE's. One of the most nearby dE's is Messier 110 (also known as NGC205), a satellite of the Andromeda galaxy. It was discovered by the French comet hunter Charles Messier in 1773 and remained the only known dwarf galaxy until, in 1944, Walter Baade confirmed NGC147 and NGC185 as members of the Local Group by resolving them into individual stars, a feat which was only possible because these dEs are very nearby galaxies. No other Local Group dE's other than these three satellites of the Andromeda galaxy have been discovered since then. In the 1950s, dE's were also discovered in the nearby Fornax and Virgo clusters.
