A STUDY OF 9 HIGH-REDSHIFT CLUSTERS OF GALAXIES - II - PHOTOMETRY, SPECTRA, AND AGES OF CLUSTER-0023-1604+4304(0423 AND CLUSTER)

We present an extensive photometric and spectroscopic study of two hig h-redshift clusters of galaxies based on data obtained from the Keek 1 0 m telescopes and the Hubble Space Telescope. The clusters Cl 0023 0423 (z = 0.84) and Cl 1604 + 4304 (z = 0.90) are part of a multiwavel ength program of Oke, Postman & Lubin to study nine candidate clusters at z greater than or similar to 0.6. Based on these observations, we study in detail both the held and cluster populations. From the confir med cluster members, we find that Cl 0023 + 0423 actually consists of two components separated by similar to 2900 km s(-1). A kinematic anal ysis indicates that the two components are a poor cluster with similar to 3 x 10(14) Mg. and a less massive group with similar to 10(13) M.. Cl 1604 + 4304 is a centrally concentrated, rich cluster at z = 0.896 7 with a velocity dispersion of 1226 km s(-1) and a mass of similar to 3 x 10(15) M.. A large percentage of the cluster members show high le vels of star formation activity. Approximately 57% and 50% of the gala xies are active in Cl 0023 + 0423 and C1 1604 + 4304, respectively. Th ese numbers are significantly larger than those found in intermediate- redshift clusters. We also observe many old, red galaxies. Found mainl y in Cl 1604 + 4304, they have spectra consistent with passive stellar evolution, typical of the populations of early-type galaxies in low- and intermediate-redshift clusters. We have calculated their ages by c omparing their spectral energy distributions to standard Bruzual & Cha riot evolutionary models. We find that their colors are consistent wit h models having an exponentially decreasing star formation rate with a time constant of 0.6 Gyr. We also observe a significant luminosity br ightening in our brightest cluster galaxies. Compared with brightest c luster galaxies at z similar to 0.1, we find a luminosity increase of similar to 1 mag in the rest M-B and similar to 0.8 mag in the rest M- V. In the field, we find that similar to 76% of the galaxies with z > 0.4 show emission-line activity. These numbers are consistent with pre vious studies. We find that an exponentially decaying star formation r ate is required to produce the observed amount of star formation for t he majority of the galaxies in our sample. A time constant of z = 0.6 Gyr appears to be optimal. We also detect several interesting galaxies at z > 1. Two of these galaxies are extremely luminous, with strong M g II: lambda 2800 absorption and Fe II resonance-line absorption. Thes e lines are so strong that we conclude that they must be generated wit hin the atmospheres of a large population of young, hot stars.