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Dwarf elliptical galaxy

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Dwarf elliptical galaxies, or dEs, are elliptical galaxy that are much smaller than others. They are classified asdE, and are quite common in galaxy groups and clusters, and are usually companions to other galaxies.

Examples

One of the most nearby Dwarf ellipticals (dEs) 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. It remained the only known dwarf elliptical galaxy until, in 1944, Walter Baade confirmed NGC147 and NGC185 as members of the Local Group by resolving them into individual stars. Resolving stars in NGC147 and NGC185 was only possible because these dEs are very nearby galaxies. In the 1950s, dEs were also discovered in the nearby Fornax and Virgo clusters.

Comparison with giant ellipticals

Dwarf elliptical galaxies have blue absolute magnitudes within the range -18 mag < M < -14 mag, fainter than giant elliptical galaxies. While the surface brightness profiles of giant elliptical galaxies are well described by de Vaucouleur's law, dEs have exponentially declining surface brightness profiles. However, both types can be well fit by the same more general law, Sersic's law, and there is a continuity of Sersic index as a function of luminosity, suggesting that dwarf and giant elliptical galaxies belong to a single sequence. Still fainter elliptical-like galaxies, called dwarf spheroidal galaxies, appear to be genuinely distinct.

Two hypotheses for origins

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. The process of coalescence is thought to lead to the present-day galaxies, and has been called "hierarchical merging". If this hypothesis is correct, dwarf galaxies may be the building blocks of today's giant galaxies. An alternative suggestion is that dEs could be the remnants of low-mass spiral galaxies that obtained a rounder shape through the action of repeated gravitational interactions with giant galaxies within a cluster. This process of changing a galaxy's morphology by interactions has been called "galaxy harassment". Evidence for this latter hypothesis has been found in the form taken by stellar disks and spiral arms of spiral galaxies. Under this alternative hypothesis the disks and arms are modified version of the original stellar disk of the transformed spiral galaxy, and similarly, small remnants of disks and arms are embedded within "harassed" dEs.

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