Butcher–Oemler Effect

The Butcher–Oemler Effect is a scientific hypothesis suggesting the cores of galaxy clusters at intermediate redshift (z ~ 0.3) contain a larger fraction of blue galaxies than do the cores of low redshift clusters. The claim was first put forward by Harvey Butcher and Augustus Oemler in a 1978 Astrophysical Journal paper.

The original Butcher–Oemler paper presents photometry of two clusters of galaxies, Cl 0024+1654 at z = 0.39 and 3C 295 at z = 0.46. These clusters are not atypical in their morphology or richness. They are both rich and centrally concentrated, rather like the nearby Coma Cluster. The surprising conclusion is that in the cores of these distant clusters more blue galaxies are observed than are observed in the cores of nearby clusters of similar richness and morphology.

Cluster galaxy "blueness" may, under certain circumstances, be used as an indicator of ongoing star formation. Astronomers have identified three spectral classes in which a significant set of blue galaxy cluster members may be categorized. The first is objects undergoing vigorous star formation with very blue colors and spectra showing emission-filled H-delta lines. Secondly, post-starburst galaxies are also observed. These are cluster members showing similarly blue colors as starburst galaxies only with moderate to strong H-delta absorption. Third are cluster members showing broad and/or high excitation line spectra, often found in active galactic nuclei. The Butcher–Oemler observations suggest at intermediate redshift a higher rate of star formation may be observed in a fraction of the galaxies in the cores of rich clusters than in the cores of rich clusters at low redshift.

The 1978 Butcher–Oemler paper sparked considerable debate. The ensuing series of Butcher–Oemler papers spans six years concluding with The Evolution of Galaxies in Clusters. V. A Study of Populations since z ~ 0.5. This paper presents photometry of 33 clusters of galaxies with redshifts varying from 0.003, the Virgo Cluster, to 0.54, the cluster Cl 0016+16. This study bolsters the conclusion put forward in their original 1978 paper, that there has been "strong, recent evolution of galaxies in clusters".

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