Giant diffuse galaxy

A giant diffuse galaxy is a supermassive, elliptical galaxy that is usually located in the centre of a cluster of galaxies. The term is often contracted to cD galaxies^[1] or BCGs (for brightest cluster galaxies).^[2] Giant diffuse galaxies are on average about 10 times more luminous than "regular" galaxies, and many times larger than a normal elliptical galaxy — some having diameters of over 6 million light years (the Milky Way, in contrast, is thought to be only around 100 thousand light years across).^[2] Giant diffuse galaxies are usually the largest galaxies in their respective clusters.

Growth

cD galaxies are believed to grow larger by accreting or merging with other galaxies,^[2] and this "cannibalising" of smaller galaxies has actually been observed. Because giant diffuse galaxies exist at the centres of galaxy clusters, there are many galaxies to "feed" on. This mode of growth leads to the overwhelming diameter and luminosity of the cDs.^[2] However, there is usually a perimeter around the cD galaxy in which no smaller galaxies exist, because they have already been accreted. There are always remains of "eaten" galaxies, appearing as a diffuse halo of gas and dust.^[2] This halo can be up to 3 million light years in diameter.^[3]

Dynamic friction

Dynamic friction is the catalytic factor in the formation of the cD galaxy at the centre of a cluster.^[4] This process begins when the motion of a large galaxy in a cluster attracts smaller galaxies into a wake behind it. This trail of smaller galaxies effectively slows the large galaxy, and, losing kinetic energy, it will gradually fall towards 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.^[4] A giant diffuse galaxy will result from this accumulation.^[4] The nuclei of merged galaxies often remain recognizable long after the mergers.

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