RECENT GALAXY MERGERS and RESIDUAL STAR FORMATION of RED SEQUENCE GALAXIES in GALAXY CLUSTERS

This study explored the Galaxy Evolution Explorer ultraviolet (UV) properties of optical red sequence galaxies in four rich Abell clusters at z ≤ 0.1. In particular, we tried to find a hint of merger-induced recent star formation (RSF) in red sequence galaxies. Using the NUV - r' colors of the galaxies, RSF fractions were derived based on various criteria for post-merger galaxies and normal galaxies. Following k-correction, about 36% of the post-merger galaxies were classified as RSF galaxies with a conservative criterion (NUV - r' ≤ 5), and that number was doubled (∼72%) when using a generous criterion (NUV - r' ≤ 5.4.). The trend was the same when we restricted the sample to galaxies within 0.5 × R₂₀₀. Post-merger galaxies with strong UV emission showed more violent, asymmetric features in the deep optical images. The RSF fractions did not show any trend along the clustocentric distance within R₂₀₀. We performed a Dressler-Shectman test to check whether the RSF galaxies had any correlation with the substructures in the galaxy clusters. Within R₂₀₀ of each cluster, the RSF galaxies did not appear to be preferentially related to the clusters' substructures. Our results suggested that only 30% of RSF red sequence galaxies show morphological hints of recent galaxy mergers. This implies that internal processes (e.g., stellar mass loss or hot gas cooling) for the supply of cold gas to early-type galaxies may play a significant role in the residual star formation of early-type galaxies at a recent epoch.