Abstract

Rest-frame Strömgren colours are presented for a large number of galaxies in rich clusters between z = 0 and z = 1. Our observations confirm a strong, rest-frame, Butcher-Oemler effect where the fraction of blue galaxies increases from 20% at z < 0.4 to 80% at z = 0.9. After isolating the red objects in each cluster we have compared the mean colour of these old, non-star forming objects with SED models from the literature as a test for passive galaxy evolution in ellipticals. We find good agreement with single burst models which predict an epoch of galaxy formation from z = 2 to 5 (Rakos et al. 1988, 1991; Rakos & Schombert 1993). Although the results demonstrate a great deal of hope for modelling the fine details of colour evolution when our samples are extended into the near- and far-IR, there are reasons to believe that galaxies become, observationally, much more complicated beyond redshifts of 1. The rate of blue colour evolution between 0.6 and 0.9 suggests that by a redshift of 1.5 it will be impossible to tell the difference between galaxies which have completed a single burst at a formation redshift of 2 or ones which are undergoing constant star formation.

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