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# 15.15 Elliptical Galaxies

Galaxies with an elliptical shape are the most common type of galaxy in the universe. Elliptical galaxies are often visually boring, with a smooth and featureless brightness profile that declines steadily from a high density of stars in the central region. These cotton balls of stars range in shape from nearly perfect spheres to elongated cigars. The vary greatly in mass and extent. The smallest elliptical span only a few hundreds of parsecs and contain just 107 solar masses of material, while the largest extend across 100 kilo parsecs and contain 1013 solar masses of material. In number of stars, then range from tens of millions (107) to more than a trillion (1012).

Hubble first described elliptical galaxies in 1936 in his book "Realm of the Nebulae." In the Hubble galaxy classification scheme, elliptical are classified according to how spherical or flattened they appear, with E0 galaxies being the roundest, and E7 being the most elongated (with the caveat that a cigar-shaped galaxy viewed end on will appear round with a small radius). Stellar motions in elliptical galaxies are largely randomly oriented, and the elliptical shape is a "frozen" snapshot a a vast number of stars all traveling on elliptical orbits, but slight deviations can be seen. So-called "boxy" elliptical appear to have cusps due to various collections of stars having similar orbits. So-called "disky" ellipticals typically have a more isotropic set of random orbits, but the system may be flattened due to some bulk system rotation. Ellipticals are mostly free of gas and dust, but about 10% have dust obscuring part of their nuclear regions and a similar percentage have detectable reservoirs of cold gas in their nuclear regions.

Galaxies at both the large and small extremes of the elliptical size range have special characteristics. The smallest dwarf spheroidal systems undergo very limited star formation, with the first generation of supernovae explosions blowing away so much gas and dust that future star formation is suppressed because there is no fuel. These tiny systems in many ways are intermediate between star clusters and galaxies. There are tens or hundreds of dwarf elliptical for every giant elliptical galaxy.

The largest elliptical galaxies are found in the hearts of galaxy clusters and are fed by the constant infall of both smaller galaxies and gas and dust that has been stripped out of galaxies. These systems are often associated with X-Ray emission from hot gas, and have larger halos than more normal sized elliptical galaxies. Like spiral galaxies, elliptical galaxies of all sizes contain dark matter that accounts for most of their mass