brightest cluster galaxy

{{Short description|Brightest galaxy within a galaxy cluster}}

Image:Abell S740.jpg that is over 450 million light-years away in the direction of the constellation Centaurus. The giant elliptical galaxy ESO 325-G004 looms large at the cluster's center. This BCG is as massive as 100 billion of our suns.]]

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), [http://adsabs.harvard.edu/abs/2004ApJ...617..879L 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 cooling flow begins due to the entropy in the galaxy falling below a key value.{{Cite web |title=Chandra :: Photo Album :: Brightest Cluster Galaxies Survey :: January 11, 2024 |url=https://chandra.harvard.edu/photo/2024/bcgs/ |access-date=2025-02-18 |website=chandra.harvard.edu}}

The study of accretion populations in BCGsMcNamara and O'Connell (1989), [http://adsabs.harvard.edu/abs/1989AJ.....98.2018M 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), [http://adsabs.harvard.edu/abs/2004ApJ...606..635M 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), [http://adsabs.harvard.edu/abs/1977ApJ...217L.125O 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), [http://adsabs.harvard.edu/abs/1984ApJ...276...26M 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), [http://adsabs.harvard.edu/abs/1985ApJ...289...18M 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), [http://adsabs.harvard.edu/abs/1998ApJ...502..141D 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 BCGs was assembled much earlier than the merging model predicts.Collins et al. (2009) [http://adsabs.harvard.edu/abs/2009Natur.458..603C Early assembly of the most massive galaxies] This is due to the lack of significant stellar mass evolution in size, luminosity, or structure over a range of redshifts, suggesting that most BCGs were assembled by redshift z~1.5−2.0, which refutes any late hierarchical assembly and emphases passive evolution. Growth primarily comes from minor mergers and gas accretion after assembly.{{Cite journal |last1=Chu |first1=A. |last2=Durret |first2=F. |last3=Márquez |first3=I. |date=2021-05-01 |title=Physical properties of brightest cluster galaxies up to redshift 1.80 based on HST data |url=https://www.aanda.org/articles/aa/full_html/2021/05/aa40245-20/aa40245-20.html |journal=Astronomy & Astrophysics |language=en |volume=649 |pages=A42 |doi=10.1051/0004-6361/202040245 |arxiv=2102.01557 |bibcode=2021A&A...649A..42C |issn=0004-6361}} Discrepancies between observations and hierarchical simulations highlight the need for model revisions to better account for early mass assembly.{{Cite journal |last1=Chu |first1=A. |last2=Sarron |first2=F. |last3=Durret |first3=F. |last4=Márquez |first4=I. |date=2022-10-01 |title=Physical properties of more than one thousand brightest cluster galaxies detected in the Canada-France-Hawaii Telescope Legacy Survey |url=https://www.aanda.org/articles/aa/full_html/2022/10/aa43504-22/aa43504-22.html |journal=Astronomy & Astrophysics |language=en |volume=666 |pages=A54 |doi=10.1051/0004-6361/202243504 |arxiv=2206.14209 |bibcode=2022A&A...666A..54C |issn=0004-6361}}

File:A green cosmic arc SDSS J1156+1911.jpg{{cite web|title=A green cosmic arc|url=https://www.spacetelescope.org/images/potw1822a/|website=www.spacetelescope.org|access-date=28 May 2018}}]]

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),[http://adsabs.harvard.edu/abs/1964ApJ...140...35M 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.