A spectroscopic study of three rich galaxy clusters at z=0.31

Abstract

We present intermediate-dispersion spectroscopy for 152 galaxies in the fields of three rich galaxy clusters at z~0.31. In each case, the fraction of blue cluster members is a factor of ~5 greater than that predicted on the basis of a morphology-density relation and colour distribution appropriate to spirals in nearby clusters. We investigate the nature of this blue galaxy excess using a spectral synthesis technique and diagnostic diagrams based on colours and line-strengths. Most of the blue excess arises from systems which have experienced a burst of star formation between 0.1 and 1.5 Gyr prior to the epoch of observation; in the remainder a substantial burst appears to be still in progress. There is no evidence for any increase in the level of normal spiral-like star-formation activity in these clusters or in the number of active nuclei. Velocity dispersions and M/L ratios have been determined for each cluster. A comparison of the distribution of populations in the high-z clusters with the morphological segregation observed in the Coma cluster, supports the view that the blue galaxies are progenitors of present-day SOs.