Morphological Butcher–Oemler Effect in the SDSS “Cut and Enhance” Galaxy Cluster Catalog

We investigated the evolution of the fractions of late-type cluster galaxies as a function of redshift using one of the largest, most uniform cluster samples available. The sample consisted of 514 clusters of galaxies in the range of $0.02 \leq z\leq 0.3$ from the Sloan Digital Sky Survey “Cut and Enhance” galaxy cluster catalog. This catalog was created using a single automated cluster-finding algorithm applied to uniform data from a single telescope, with accurate CCD photometry, thus minimizing the selection biases. We used four independent methods to analyze the evolution of the late-type galaxy fraction. Specifically, we selected late-type galaxies based on each of the rest-frame $g-r$ color, the $u-r$ color, galaxy profile fitting, and the concentration index. The first criterion corresponds to that used in classical Butcher–Oemler analyses. The last two criteria are more sensitive to the morphological type of galaxies. In all four cases, we find an increase in the fraction of late-type galaxies with increasing redshift, significant at the 99.9% level. The results confirm that cluster galaxies do change colors with redshift (the Butcher–Oemler effect) and, in addition, they change their morphology to later-type toward a higher redshift — indicating a morphological equivalence of the Butcher–Oemler effect. We also found a tendency of richer clusters to have lower fractions of late-type galaxies. This trend is consistent with a ram-pressure stripping model, where galaxies in richer clusters are affected by stronger ram pressure due to the higher temperature of clusters.