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A photometric and spectroscopic survey of a cluster of galaxies at z = 0. 327 is presented. This is the first cluster we have studied from our sample of five distant, high X-ray luminosity clusters, which have been selected from the Einstein Observatory Medium Sensitivity Survey Catalog. Studies by others of optically selected clusters at z > 0. 2 have led to the important conclusion that the galaxy population in such clusters is markedly different than in comparable, nearby clusters. Our study of the galaxy population in an X-ray-selected sample will allow us to assess the importance of optical selection biases. In addition, the use of different selection criteria may help to clarify the causes of galaxy evolution in clusters. We have obtained V, R, and I photometry of 288 stars and galaxies in a region 5. 7' x 6. 5' which is approximately centered on the dominant galaxy at the cluster center. The photometry is complete to rest band MV_ = - 19. 5 (19 kpc diameter aperture). We have obtained the spectra of 70 galaxies which lie within ~2' (~R₃0_) of the cluster center, to a magnitude limit of MV_~ - 20. 5; ~90% of these galaxies are cluster members. The large velocity dispersion of the cluster, 1125^+126^_-94_ km s^-1^, is consistent with its high X-ray luminosity, ~7 x 10⁴4^ ergs s^-1^ (0. 2-4. 5 keV). We find evidence for evolution of the galaxy population as compared with clusters at the present epoch. Approximately 17% of the cluster members have "E + A" spectra, with strong Balmer absorption lines, the normal absorption features of E/S0 galaxies, and no emission lines. In comparable nearby clusters, on the other hand only ~1% of the galaxies have E + A spectra. Approximately 10% of the cluster members have emission-line spectra compared with ~5% in present epoch clusters. The total "active" population, defined as the sum of the E + A and emission- line populations, represents ~27% of the population in the cluster core. These results are consistent with the size of the active population found in optically selected samples of distant clusters at similar redshifts.
Fabricant et al. (Fri,) studied this question.