Post-merger Signatures of Red-sequence Galaxies in Rich Abell Clusters
  at $z\lesssim 0.1$
  We have investigated the post-merger signatures of red-sequence galaxies in
rich Abell clusters at $z \lesssim$ 0.1: A119, A2670, A3330 and A389. Deep
images in u', g', r' and medium-resolution galaxy spectra were taken using
MOSAIC 2 CCD and Hydra MOS mounted on a Blanco 4-m telescope at CTIO.
Post-merger features are identified by visual inspection based on asymmetric
disturbed features, faint structures, discontinuous halo structures, rings and
dust lanes. We found that ~ 25% of bright (M_r < -20) cluster red-sequence
galaxies show post-merger signatures in four clusters consistently. Most (~
71%) of the featured galaxies were found to be bulge-dominated, and for the
subsample of bulge-dominated red-sequence galaxies, the post-merger fraction
rises to ~ 38%. We also found that roughly 4% of bulge-dominated red-sequence
galaxies interact (on-going merger). A total of 42% (38% post-merger, 4%
on-going merger) of galaxies show merger-related features. Compared to a field
galaxy study with a similar limiting magnitude (van Dokkum 2005), our cluster
study presents a similar post-merger fraction but a markedly lower on-going
merger fraction. The merger fraction derived is surprisingly high for the high
density of our clusters, where the fast internal motions of galaxies are
thought to play a negative role in galaxy mergers. The fraction of post-merger
and on-going merger galaxies can be explained as follows. Most of the
post-merger galaxies may have carried over their merger features from their
previous halo environment, whereas interacting galaxies interact in the current
cluster in situ. According to our semi-analytic calculation, massive cluster
haloes may very well have experienced tens of halo mergers over the last 4-5
Gyr; post-merger features last that long, allowing these features to be
detected in our clusters today. (Abridged)
