Evidence for systematic evolution in the properties of galaxies in distant clusters

Abstract

This paper investigates the evolution of galaxies as a function of look-back time, primarily using early-type galaxies in rich cluster environments. We demonstrate that, by selecting distant galaxies in the near-infrared, representative samples containing approximately the same proportion of galaxy classes can be constructed independently of redshift, thus avoiding biases introduced by optical selection criteria. Using 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 yielding the required high-precision photometry of cluster members, are discussed. Additional optical CCD photometry allows us to quantify colour evolution as a function of redshift for the early-type members to z ~ 1. We detect a clear and systematic trend with redshift in the optical-infrared colours of red cluster galaxies. Specifically, by z ~ 0.9 there are no cluster galaxies as red as present-day ellipticals. The detected evolution is monotonic with redshift and consistent with the passive ageing of stellar populations formed before z ≈ 2. Moreover, the uniformity in these trends among our clusters suggests that early-type galaxies are coeval and form a remarkably homogeneous population. The evolution we find is both more well-behaved and less prominent than that found for powerful radio galaxies, suggesting that the latter sources may not be representative giant elliptical galaxies.