Galaxy Evolution in Distant, X-Ray Selected Clusters of Galaxies. II. 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 ~ 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; 53 of these are cluster members. The x-ray luminosities of Cl 0303+1658 and Cl 0302+1717 (0.2-4.5 keV) are ~5 X 10^44^ and ~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 ~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 ~ 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_>^bar^ > 8 A, are so far absent in our x-ray selected sample, yet they constitute 25% of the spectroscopically active population in Dressler and Gunn's optically selected sample.