Brightest cluster galaxies {BCGs) are the brightest and most massive galaxies in the universe, emitting purely photospheric light. They are generally elliptical galaxies which lie close to the host galaxy cluster kinematic centre, and except for those undergoing major mergers, they lie at the bottom of the cluster potential well and close to the X-ray peak emission.

Their small variance in luminosity, abundance and bright apparent magnitude make them excellent standard candles for distance determination.

Formation scenarios for BCGs include:

• Cooling flow — Star formation from the central cooling flow in high density cooling centres of X-ray cluster halos.

The study of accretion populations in BCGs carried out by McNamara and O’Connell (1989) has cast doubt over this theory and Motl et al. (2004) have since seen no evidence of cooling flows in radiative cooling clusters. The two remaining theories exhibit healthier prospects.

• Galactic cannibalism — Galaxies sink to the centre of the cluster due to dynamical friction and tidal stripping.
• Galactic merger — Rapid galactic mergers between several galaxies take place during cluster collapse.

It is possible to differentiate the cannibalism model from the merging model by considering the formation period of the BCGs. In the cannibalism model, there are numerous small galaxies present in the evolved cluster, where in the merging model, a hierarchical cosmological model is expected due to the collapse of clusters. The origin of the merging model is much earlier than the previous two.

BCGs are divided into various classes of galaxies. Giant ellipticals (gE), D galaxies and cD galaxies, defined by Matthews, Morgan and Schmidt (1964). cD and D both exhibit an extended diffuse envelope surrounding an elliptical-like nucleus akin to regular ellipticals (Tonry 1987), the light profile of BCGs is described by a de Vaucouleurs surface brightness law (de Vaucouleurs 1948).

Sources

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• de Vaucouleurs, G. 1948 Annales d'Astrophysique, vol. 11, p.247
• Dubinski, J. 1998 The Astrophysical Journal, vol. 502, p.141
• Matthews, T., Morgan, W. W., & Schmidt, M. 1964 The Astrophysical Journal, vol. 140, p.35
• McNamara, B. R. & O'Connell, R. W. 1989 The Astronomical Journal, vol. 98, p.2018
• Motl, Patrick M.; Burns, Jack O.; Loken, Chris; Norman, Michael L., & Bryan, Greg 2004 The Astrophysical Journal, vol. 606, p.635
• Tonry, J. L. 1987 International Astronomical Union Symposium, vol. 127, p.89

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Extragalactic astronomy | Galaxies