cD galaxy

A cD galaxy is an exceptionally luminous, massive elliptical galaxy surrounded by an extended stellar envelope. It is usually found near the center of a galaxy cluster.

Galaxy cluster: A gravitationally bound system containing many galaxies, hot gas, and dark matter. cD galaxies typically occupy the central regions of these large systems.
Galaxy merger: An interaction in which two or more galaxies combine through gravity into a larger system. Repeated mergers can build the enormous stellar mass of a cD galaxy.
NGC 6166: A giant elliptical galaxy at the center of the Abell 2199 galaxy cluster. Its extended stellar envelope makes it a classic cD galaxy.
Intracluster medium: The hot, diffuse plasma that fills the space between galaxies in a cluster. Its gravitational and thermal environment affects the growth and surroundings of central cD galaxies.
Elliptical galaxy: A galaxy with a smooth, rounded or elongated shape and little organized disk structure. cD galaxies are giant ellipticals distinguished by their unusually extended envelopes.
Dynamical friction: A drag-like gravitational effect that slows a massive object moving through a background of matter. It can draw cluster galaxies inward, making mergers with the central galaxy more likely.
IC 1101: A giant elliptical galaxy near the center of the Abell 2029 galaxy cluster. It is often cited as an exceptionally large example of a cD galaxy.
Cooling flow problem: The discrepancy between predicted rapid cooling of cluster gas and the smaller amounts of cold gas observed. Cooling and feedback in cluster centers influence star formation around cD galaxies.
Stellar halo: A diffuse, extended distribution of stars surrounding a galaxy's brighter central body. The cD envelope is an especially prominent stellar halo.
Tidal stripping: The removal of matter from an object by the gravitational field of a more massive neighbor. Cluster tides can strip stars from passing galaxies and add them to the cD envelope.

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