Poor groups around strong gravitational lenses

Poor groups are common and interactive environments for galaxies, and thus are important laboratories for studying galaxy evolution. Unfortunately, little is known about groups at z 0.1, because of the difficulty in identifying them in the first place. Here we present results from our ongoing survey of the environments of strong gravitational lenses, in which we have so far discovered six distant (z 0.5) groups of galaxies. As in the local Universe, the highest velocity dispersion groups contain a brightest member spatially coincident with the group centroid, whereas lower-dispersion groups tend to have an offset brightest group galaxy. This suggests that higher-dispersion groups are more dynamically relaxed than lower-dispersion groups and that at least some evolved groups exist by z ∼ 0.5. We also compare the galaxy and hot gas kinematics with those of similarly distant clusters and of nearby groups.