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ABSTRACT  We present quantitative morphology measurements of a sample of optically selected group galaxies at  0.3 &lt;  z  &lt; 0.55  using the  Hubble Space Telescope  ( HST ) Advanced Camera for Surveys (ACS) and the  gim2d  surface brightness fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) and follow‐up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at  0.05 &lt;  z  &lt; 0.12 . We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disc‐dominated (B/T &lt; 0.2) galaxies in the group samples. At fixed luminosity,   z = 0.4  groups have  ∼5.5 ± 2  per cent fewer disc‐dominated galaxies than the field, while by   z = 0.1  this difference has increased to  ∼19 ± 6  per cent. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disc population. At both redshifts, the discs of group galaxies have similar scaling relations and show similar median asymmetries as the discs of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge‐dominated galaxies is  6 ± 3  per cent higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at   z = 0.4  and 0 using the semi‐analytic galaxy catalogues of Bower et al. This model accurately reproduces the B/T distributions of the group and field at   z = 0.1 . However, the model does not reproduce our finding that the deficit of discs in groups has increased significantly since   z = 0.4 .

Evolution in the discs and bulges of group galaxies since z = 0.4