EVIDENCE FOR SYSTEMATIC EVOLUTION IN THE PROPERTIES OF GALAXIES IN DISTANT CLUSTERS

This paper investigates the evolution of galaxies as a function of loo k-back time, primarily using early-type galaxies in rich cluster envir onments. We demonstrate that, by selecting distant galaxies in the nea r-infrared, representative samples containing approximately the same p roportion of galaxy classes can be constructed independently of redshi ft, thus avoiding biases introduced by optical selection criteria. Usi ng this method, we construct a sample of 180 galaxies selected at near -infrared wavelengths in the fields of 10 rich clusters with 0.5 < z < 0.9. Observing and data processing techniques, with infrared arrays y ielding the required high-precision photometry of cluster members, are discussed. Additional optical CCD photometry allows us to quantify co lour evolution as a function of redshift for the early-type members to z approximately 1. We detect a clear and systematic trend with redshi ft in the optical-infrared colours of red cluster galaxies. Specifical ly, by z approximately 0.9 there are no cluster galaxies as red as pre sent-day ellipticals. The detected evolution is monotonic with redshif t and consistent with the passive ageing of stellar populations formed before z almost-equal-to 2. Moreover, the uniformity in these trends among our clusters suggests that early-type galaxies are coeval and fo rm a remarkably homogeneous population. The evolution we find is both more well-behaved and less prominent than that found for powerful radi o galaxies, suggesting that the latter sources may not be representati ve giant elliptical galaxies.