A cluster of galaxies at z = 0.9
Date
1990
Authors
Ross, Gordon Chattan
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Abstract
We present a study of faint galaxies in the field surrounding the z = 0.90 radio galaxy, 3C 217. Deep broadband and narrowband CCD images were obtained by Pritchet and Hartwick (1987) at the prime focus of the Canada-France-Hawaii 3.6 m telescope and limiting magnitudes of B = 25.5, R = 25 and J = 24 were achieved. Number counts indicate two to four times the number of galaxies expected for a field sample and the distribution of excess counts provides an acceptable fit to a Schechter function at the redshift of 3C 217. The colours and spectral energy distributions of the brightest objects in I were compared with predicted values, which permitted the differentiation of field and cluster galaxies. Many of these cluster galaxies show strong evolution with B - R and R - I colours indicating star formation rates of µ ~ 0.5 in Bruzual's exponential models. However, there are strong K corrections and selection effects that limit the detectability of faint elliptical galaxies. After the strength of these effects were estimated, the sample was transformed to an absolute V magnitude system. The sample shows a fractional content of 20- 30% evolving galaxies to a limiting absolute magnitude of M v = -19.8 (H0 = l00 ,q0 = 0.0). This is comparable to the blue fraction found in the high redshift clusters (z ~ 0.5) in the Butcher-Oemler (1984) sample, but does not appear compatible with the extension of the Butcher-Oemler effect out to z = 0.9. Observations 1.0 mag fainter in R would suffice to confirm the elliptical content.