Merger relics of cluster galaxies
  Context. Sheen and collaborators recently found that a surprisingly large
portion (38%) of massive early-type galaxies in heavy clusters show strong
merger-related disturbed features. This contradicts the general understanding
that massive clusters are hostile environments for galaxy mergers. Considering
the significance of mergers in galaxy evolution, it is important to understand
this. Aims. We aim to present a theoretical foundation that explains galaxy
mergers in massive clusters. Methods. We used the N-body simulation technique
to perform a cosmological-volume simulation and derive dark-halo merger trees.
Then, we used the semi-analytic modeling technique to populate each halo with
galaxies. We ran hydrodynamic simulations of galaxy mergers to estimate the
lifetime of merger features for the imaging condition used by Sheen and
collaborators. We applied this merger feature lifetime to our semi-analytic
models. Finally, we counted the massive early-type galaxies in heavy model
clusters that would show strong merger features. Results. While there still are
substantial uncertainties, our preliminary results are remarkably close to the
observed fraction of galaxies with merger features. Key ingredients for the
success are twofold: firstly, the subhalo motion in dark haloes has been
accurately traced, and, second, the lifetime of merger features has been
properly estimated. As a result, merger features are expected to last very long
in cluster environments. Many massive early-type galaxies in heavy clusters
therefore show merger features not because they experience mergers in the
current clusters in situ, but because they still carry their merger features
from their previous halo environments. Conclusions. Investigating the merger
relics of cluster galaxies is potentially important, because it uniquely allows
us to backtrack the halo merger history.
