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Studying Galaxy Clusters at High Redshifts (1Mar94) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 37, 1 March 1994) Observations of high redshift clusters of galaxies have resulted in tantalizing insights into the evolution of galaxies in rich environments and of the most massive collapsed peaks in the cosmological matter fluctuation spectrum. The advent of modern, large-format optical and infrared array detectors and multi-object spectrographs has provided a wealth of new data on distant clusters and the spectral properties of the galaxies they contain. The number of known high-z clusters with published redshifts is remarkably small, however, primarily because spectroscopy on faint galaxies is so challenging. Nearly all observational effort has been concentrated on the same few systems, particularly at z > 0.5 where only a handful of clusters are known (mostly from deep photographic surveys). The incompletenesses and biases which may affect such catalogs at their limits may have unknown effects on our understanding of these most distant clusters. [Figure not included] K-band image of the z = 1.169 radio source 3C210, and associated cluster. For his thesis at the University of California, Berkeley, Mark Dickinson (now at STScI) worked to expand the available sample of high redshift clusters and circumvent some of the usual optical selection biases by identifying and studying clusters around distant, powerful radio galaxies. Using the 2.1-m and 4-m telescopes at KPNO and the 1.5-m and 4-m telescopes at CTIO, Dickinson surveyed the environments of radio galaxies as well as known, optically selected clusters out to z = 0.8. Imaging through a large complement of narrow and intermediate bandwidth filters, he measured low-resolution "spectra" of complete samples of hundreds of galaxies, identifying cluster members and characterizing their spectral properties. S. Adam Stanford (IPAC/JPL), Dickinson and Peter Eisenhardt (JPL) have expanded on this work by extending these measurements into the near-infrared, taking advantage of the new, large-format IR detector technology used in the SQIID, IRIM and OSIRIS imagers to obtain deep wide-field J, H and K band imaging photometry for 18 clusters from nearby Coma out to z = 0.7. Dickinson and Eisenhardt have recently begun to extend this work to still higher redshifts, beyond the limits of available optically selected samples. Using the new, improved IRIM with its NICMOS-3 array on its first night of regular operation on the KPNO 4-m, they imaged two powerful radio galaxies at z = 1.2 in the near infrared, taking advantage of the camera's wide field of view to survey a region ~ 1.5 Mpc on a side around each target. Remarkably, they found that both radio galaxies are surrounded by large, centrally concentrated swarms of extremely red galaxies - so red, in fact, that their R-K and J-K colors match those expected for present-day (z = 0) ellipticals redshifted out to z = 1.2 with little or no spectral evolution. While there is as yet no spectroscopic confirmation of these galaxies' redshifts, the guilt-by-association is strong. Their colors are too red for them to be "normal" galaxies at z < 1 without invoking strong extinction; only the large k-correction expected for quiescent ellipticals at high redshift can account for them. Such red objects are not unknown in deep field surveys, but they are rare compared to the dramatic over-densities found near these radio sources. The red colors render these clusters virtually invisible in deep optical images, suggesting that such systems might not be recognized in even the deepest optical surveys. Perhaps more importantly, if the colors accurately reflect the stellar populations in these galaxies, their last major episode of star formation must have occurred several billion years prior to z = 1.2. For closed cosmologies, this strains the limits on the age of the universe unless the Hubble constant is small. As their survey continues, Dickinson and Eisenhardt plan to employ this promising technique to expand this sample of "normal" cluster galaxies and extend it to still higher redshifts.
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