The type-cD galaxy (also cD-type galaxy, cD galaxy) is a galaxy morphology classification, a subtype of type-D giant elliptical galaxy. Characterized by a large halo of stars, they can be found near the centres of some rich galaxy clusters. They are also known as supergiant ellipticals or central dominant galaxies.
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Characteristics
The cD-type is a classification in the Yerkes galaxy classification scheme, one of two Yerkes classifications still in common use, along with D-type. The "c" in "cD" refers to the fact that the galaxies are very large, hence the adjective supergiant, while the "D" refers to the fact that the galaxies appear diffuse. A backformation of "cD" is frequently used to indicate "central Dominant galaxy". cD's are also frequently considered the largest galaxies.
cD galaxies are similar to lenticular galaxies (S0) or elliptical galaxies (E#), but many times larger, some having envelopes that exceed one million light years in radius. They appear elliptical-like, with large low surface brightness envelopes. It is currently thought that cD's are the result of galaxy mergers. Some cD's have multiple galactic nuclei. cD galaxies are one of the types frequently found to be the brightest cluster galaxy (BCG) of a cluster. Many fossil group galaxies are similar to cD BCG galaxies, leading some to theorize that the cD results from the creation of a fossil group, and then the new cluster accumulating around the fossil group. However, cD's themselves are not found as field galaxies, unlike fossil groups. cD's form around 20% of BCGs.
Growth
cD galaxies are believed to grow via mergers of galaxies that spiral in to the center of a galaxy cluster, a theory first proposed by Herbert J. Rood in 1965. This "cannibalistic" mode of growth leads to the large diameter and luminosity of the cD's. The second-brightest galaxy in the cluster is usually under-luminous, a consequence of its having been "eaten". Remains of "eaten" galaxies sometimes appear as a diffuse halo of gas and dust. This halo can be up to 3 million light years in diameter. It is estimated that the cD galaxy alone contributes 1-7%, depending on the cluster mass, of the total baryon mass within 12.5 virial radii.
Dynamical friction
Dynamical friction is believed to play an important role in the formation of cD galaxies at the centres of galaxy clusters. This process begins when the motion of a large galaxy in a cluster attracts smaller galaxies and dark matter into a wake behind it. This over-density follows behind the larger galaxy and exerts a constant gravitational force on it, causing it to slow down. As it loses kinetic energy, the large galaxy gradually spirals toward the centre of the cluster. Once there, the stars, gas, dust and dark matter of the large galaxy and its trailing galaxies will join with those of other galaxies who preceded them in the same fate. A giant or supergiant diffuse or elliptical galaxy will result from this accumulation. The centers of merged or merging galaxies can remain recognizable for long times, appearing as multiple "nuclei" of the cD galaxy.
cD clusters
Type-cD galaxies are also used to define clusters. A galaxy cluster with a cD at its centre is termed a "cD cluster" or "cD galaxy cluster" or "cD cluster of galaxies".