Substructure in the Most Massive GEEC Groups: Field-like Populations in
  Dynamically Active Groups
  The presence of substructure in galaxy groups and clusters is believed to be
a sign of recent galaxy accretion and can be used not only to probe the
assembly history of these structures, but also the evolution of their member
galaxies. Using the Dressler-Shectman (DS) Test, we study substructure in a
sample of intermediate redshift (z ~ 0.4) galaxy groups from the Group
Environment and Evolution Collaboration (GEEC) group catalog. We find that 4 of
the 15 rich GEEC groups, with an average velocity dispersion of ~525 km s-1,
are identified as having significant substructure. The identified regions of
localized substructure lie on the group outskirts and in some cases appear to
be infalling. In a comparison of galaxy properties for the members of groups
with and without substructure, we find that the groups with substructure have a
significantly higher fraction of blue and star-forming galaxies and a parent
colour distribution that resembles that of the field population rather than the
overall group population. In addition, we observe correlations between the
detection of substructure and other dynamical measures, such as velocity
distributions and velocity dispersion profiles. Based on this analysis, we
conclude that some galaxy groups contain significant substructure and that
these groups have properties and galaxy populations that differ from groups
with no detected substructure. These results indicate that the substructure
galaxies, which lie preferentially on the group outskirts and could be
infalling, do not exhibit signs of environmental effects, since little or no
star-formation quenching is observed in these systems.
