GALAXY EVOLUTION IN DISTANT, X-RAY SELECTED CLUSTERS OF GALAXIES .2. CL 0303+1658 AND CL 0303+1717

We present a photometric and spectroscopic survey of two clusters of galaxies at z=0.42. These are members of a sample of five distant, high x-ray luminosity clusters of galaxies, which have been selected from the Einstein Medium Sensitivity Survey Catalog. Our objective is to compare the galaxy population in these x-ray selected clusters to the populations in optically selected clusters, which show strong evolution. We have obtained R and I photometry complete to R approximately 22.3 in a 5.7' X 6.5' field centered on each cluster. In addition, we have obtained the spectra of 69 galaxies in the direction of the two clusters; 5 3 of these are cluster members. The x-ray luminosities of Cl 0303 + 1658 and Cl 0302 + 1717 (0.2-4.5 keV) are approximately 5 X 10(44) and approximately 4 X 10(44) ergs-1, and their velocity dispersions are 921(-123)+192 and 821(-94)+137 km s-1, respectively. The evolution of the galaxy population observed in optically selected clusters at z approximately 0.4 (as compared with clusters at the present epoch) is also found in these x-ray selected clusters. This confirms the results that we presented in a previous paper for Cl 1358+6245 at z=0.33. In both optically and x-ray selected clusters at z approximately 0.4, 20-30% of the galaxies have spectra with strong Balmer absorption lines or emission lines, consistent with recent star formation activity. We identify one possible difference between the optically and x-ray selected clusters; galaxies with very strong Balmer absorption lines, EW (H(delta) + H(gamma)) > 8 angstrom, are so far absent in our x-ray selected sample, yet they constitute approximately 25% of the spectroscopically active population in Dressler and Gunn's optically selected sample.