Photometry of distant clusters of galaxies : evolutionary features in the cluster 2158+0351 (z = 0.45).

Results of detailed three-colour photometry of the distant cluster 21 58 + 035 (z = 0.445) are presented. A photometric catalogue has been produced using the INVENTORY package, for which we give an estimate of completeness and photometry errors. In total we measured Gunn g, r, i magnitudes for 294 objects, down to r = 23.5, with a typical error of 0.1 mag. On the basis of a statistical analysis, about 60 per cent of these are suggested to be cluster members. Luminosity functions ar shown along with colour-magnitude and two-colour diagrams. We find that the bright- cluster galaxy population is dominated by ellipticals, while a population of spirals becomes relevant at the faint luminosities. Such an overabundance of faint spiral leads us to calculate a high fraction of blue objects, according to the Butcher-Oemler effect. We find f_B_ = 0.3 +/- 0.13. We evidence possible photometric effects on magnitudes and colours due to galactic evolution. In particular, elliptical galaxies in the cluster are recognized to be bluer than present-day descendants, and the first-rank galaxy is intrinsically brighter by about 0.8 mag. An independent estimate of the redshift of the cluster, through photometric properties of its galaxy population, - attempted, deriving z = 0.44 +/- 0.05. This is obtained taking into account appropriate evolutionary k-corrections from population-synthesis models. The importance of such a correction when studying high redshift (young) galaxies is stressed.