5 v 1 1 2 M ay 1 99 8 A Study of Nine High - Redshift Clusters of Galaxies

We present an extensive photometric and spectroscopic study of two high-redshift clusters of galaxies based on data obtained from the Keck 10m telescopes and the Hubble Space Telescope. The clusters CL0023+0423 (z = 0.84) and CL1604+4304 (z = 0.90) are part of a multi-wavelength program to study nine candidate clusters at z > ∼ 0.6 (Oke, Postman & Lubin 1998). Based on these observations, we study in detail both the field and cluster populations. From the confirmed cluster members, we find that CL0023+0423 actually consists of two components separated by ∼2900 km s. A kinematic analysis indicates that the two components are a poor cluster with ∼3 ×10 M⊙ and a less massive group with ∼10 13 M⊙. CL1604+4304 is a centrally concentrated, rich cluster at z = 0.8967 with a velocity dispersion of 1226 km s and a mass of ∼3 ×10 M⊙. A large percentage of the cluster members show high levels of star formation activity. Approximately 57% and 50% of the galaxies are active in CL0023+0423 and CL1604+4304, respectively. These numbers are significantly larger than those found in intermediate-redshift clusters (Balogh et al. 1997). We also observe many old, red galaxies. Found mainly in CL1604+4304, they have spectra consistent with passive stellar evolution, typical of the populations of early-type galaxies in low and intermediateredshift clusters. We have calculated their ages by comparing their spectral energy distributions to standard Bruzual & Charlot (1995) evolutionary models. We find that their colors are consistent with models having an exponentially decreasing star formation rate with a time constant of 0.6 Gyr. We also observe a significant luminosity brightening in our brightest cluster galaxies. Compared to brightest cluster galaxies at z ∼ 0.1, we find a luminosity increase of ∼ 1 mag in the rest MB and ∼ 0.8 mag in the rest MV . In the field, we find that ∼ 76% of the galaxies with z > 0.4 show emission line activity. These numbers are consistent with previous studies (e.g. Hammer et al. 1997). We find that an exponentially decaying star formation rate is required to produce the observed amount of star formation for the majority of the galaxies in our sample. A time constant of τ = 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 MgIIλ2800 absorption and FeII resonance line absorption. These lines are so strong that we conclude that they must be generated within the atmospheres of a large population of young, hot stars.