Ellipticals

Ellipticals, or elliptical galaxies, are a class of galaxies with smooth, ellipsoidal light distributions and no spiral arms. They range from nearly circular E0 to elongated E7 and are common in dense environments such as galaxy clusters, where giant ellipticals frequently reside at centers.

Their light profiles are well described by de Vaucouleurs or Sersic laws, with the surface brightness falling roughly as r^(1/4) in many cases. Giant ellipticals often have extended halos, while dwarf ellipticals (dEs) are smaller and fainter members of the same family. The Sersic index for ellipticals typically lies around n ≈ 4, though variations exist across the population.

Ellipticals typically host older, metal-rich stars and contain little cold gas or dust, leading to minimal ongoing star formation. They appear red and have high stellar velocity dispersions, dominated by random motions, though some show rotation or boxy/disky isophotes.

Formation is explained mainly by hierarchical assembly through mergers and interactions, with rapid early star formation in some cases. Mergers, gas accretion, and feedback from supermassive black holes help quench star formation and build spheroidal shapes. While mergers are a central component of current models, the relative importance of major versus minor mergers and the role of environment are ongoing research topics.

Ellipticals are common in clusters and groups and tend to lie on the red sequence in color–magnitude space. They span a range of masses, with the most massive members occupying cluster centers. Notable nearby examples include giant ellipticals such as M87 and NGC 4472.