The morphology-density relation in X-ray bright galaxy groups
  We have examined the morphological make-up of X-ray bright groups. The brighter galaxies in these groups exhibit clear morphology-density and morphology-radius relations. The group morphology-density relation is offset from the cluster relation in the sense that at a given surface density, X-ray bright groups have a lower spiral fraction. After correcting for projection effects the morphology-3D density relation is still shifted towards fewer spirals for a given 3D density, in comparison with clusters. A simple model which corrects the group data for the effects of projection and for the expected higher merging rate in groups, brings the morphology-density relation into good agreement with that of clusters, suggesting that the relation may be driven by two-body interactions. The fraction of S0 galaxies in these X-ray bright groups is at least as high as that observed in nearby clusters. Given the low velocity dispersion of groups, this indicates that ram pressure stripping is not the dominant mechanism for S0 formation. 
Stephen F. Helsdon and Trevor J. Ponman