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A brightest cluster galaxy (BCG) is defined as the brightest galaxy in a cluster of galaxies. BCGs include the most massive galaxies in the universe. They are generally elliptical galaxies which lie close to the geometric and kinematical center of their host galaxy cluster, hence at the bottom of the cluster potential well. They are also generally coincident with the peak of the cluster X-ray emission.〔Lin and Mohr (2004), ( K-band Properties of Galaxy Clusters and Groups: Brightest Cluster Galaxies and Intracluster Light )〕 Formation scenarios for BCGs include: *Cooling flow—Star formation from the central cooling flow in high density cooling centers of X-ray cluster halos. The study of accretion populations in BCGs〔McNamara and O’Connell (1989), ( Star formation in cooling flows in clusters of galaxies )〕 has cast doubt over this theory and astronomers have seen no evidence of cooling flows in radiative cooling clusters.〔Motl et al. (2004), ( Formation of Cool Cores in Galaxy Clusters via Hierarchical Mergers )〕 The two remaining theories exhibit healthier prospects. *Galactic cannibalism—Galaxies sink to the center of the cluster due to dynamical friction and tidal stripping.〔J. Ostriker and M. Hausman (1977), ( Cannibalism among the galaxies - Dynamically produced evolution of cluster luminosity functions )〕 *Galactic merger—Rapid galactic mergers between several galaxies take place during cluster collapse.〔D. Merritt (1984), ( Relaxation and tidal stripping in rich clusters of galaxies. II - Evolution of the luminosity distribution )〕 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, whereas in the merging model, a hierarchical cosmological model is expected due to the collapse of clusters. It has been shown that the orbit decay of cluster galaxies is not effective enough to account for the growth of BCGs.〔D. Merritt (1985), (Relaxation and tidal stripping in rich clusters of galaxies. III Growth of a massive central galaxy )〕 The merging model is now generally accepted as the most likely one,〔J. Dubinski (1998), ( The Origin of the Brightest Cluster Galaxies )〕 but recent observations are at odds with some of its predictions. For example, it has been found that the stellar mass of BCG's was assembled much earlier than the merging model predicts.〔Collins et al. (2009) (Early assembly of the most massive galaxies )〕 BCGs are divided into various classes of galaxies: giant ellipticals (gE), D galaxies and cD galaxies.〔Matthews, T. A., Morgan, W. W. and Schmidt, M. (1964),( A Discussion of Galaxies Identified with Radio Sources )〕 cD and D galaxies both exhibit an extended diffuse envelope surrounding an elliptical-like nucleus akin to regular elliptical galaxies. The light profiles of BCGs are often described by a Sersic surface brightness law, a double Sersic profile or a de Vaucouleurs law. The different parametrizations of the light profile of BCG's, as well as the faintness of the diffuse envelope lead to discrepancies in the reported values of the sizes of these objects. == References == 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「brightest cluster galaxy」の詳細全文を読む スポンサード リンク
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