2004ApJ...617L.111T

We find that the fraction of early-type galaxies in poor groups (containing from four to 10 members) is a weakly increasing function of the number of the group members and is about 2 times higher than in a sample of isolated galaxies. We also find that the group velocity dispersion increases weakly with the fraction of early-type galaxies. Early-type galaxies in poor groups are brighter in the near-infrared with respect to isolated ones by ΔM_K∼0.75 mag, and to a lesser degree also in the blue (ΔM_B∼0.5 mag). We also find early-type galaxies in groups to be redder than those in the field. These findings suggest that the formation history for early-type galaxies in overdense regions is different from that in underdense regions, and that their formation in groups is triggered by merging processes.