Properties of CD galaxies.

cD galaxies are the most luminous galaxies in the universe. They are characterized by a surface brightness profile that falls off more slowly with radius than most elliptical galaxies. In most respects D galaxies are a continuous extrapolation from other ellipticals: their M/L and their colors are comparable to other ellipticals, their inner parts are fitted by an r exp 1/4 law, and they follow the same relation between L and sigma. On the other hand, their luminosity is too bright to be consistent with the luminosity function of other ellipticals and they are always found at the center of a cluster of other galaxies. Being at the center of a cluster of galaxies often endows D galaxies with a very faint, very extended halo of luminosity and multiple nuclei, but these are more properly associated with the cluster than the D galaxy itself. The connection between the formation of cD galaxies and the formation of clusters remains a mystery. It is still unresolved whether cDs are a byproduct of cluster evolution, whether they formed in parallel with clusters, or whether primeval D are galaxies the seed around which clusters accreted.