Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aas...20114801g&link_type=abstract
American Astronomical Society Meeting 201, #148.01; Bulletin of the American Astronomical Society, Vol. 35, p.573
Astronomy and Astrophysics
Astronomy
Scientific paper
The Butcher-Oemler effect is an enigmatic observable. Our literature survey of 238 clusters with published redshifts and blue fractions demonstrates an overall trend for higher redshift clusters to have a higher blue fraction than lower redshift systems. While studying blue fraction trends as a function of cluster richness, we determined that groups (R < 0) show no definitive B-O effect and in general have a large blue fraction. We propose that the evolution of galaxies in groups more closely resembles the evolution of field galaxies than that of cluster galaxies. Very rich clusters (R = 4, 5) also show little or no blue fraction evolution with redshift and have a consistently low blue fraction. Clusters with intermediate redshifts show the B-O effect, with lower richness clusters having a steeper increase in blue fraction with redshift than higher redshift clusters.
We believe that in clusters, the observed dependence of the B-O effect on both redshift and cluster richness is a reflection of field galaxy infall rates changing with redshift and the ability of the cluster environment to trigger and/or suppress star formation, as well as the cluster's size. A rich red cluster can only minimally be blue enhanced by field galaxy infall. Additionally, the primary mechanisms for triggering star formation - interactions, and tidal effects - work most effectively in systems with a low velocity dispersions and a diffuse ICM; e.g. in lower richness clusters. Ram pressure stripping, which truncates star formation, is most effective in high mass systems. Thus, star formation is most likely to be triggered and last in lower richness clusters, while in high richness clusters, what star formation does occur is quickly extinguished. The change in blue fraction for clusters of similar richnesses at different epochs is explained by the change in field galaxy infall rates with time.
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