The extension of the Hubble diagram. I. New redshifts and BVR photometry of remote cluster galaxies, and an improved richness correction

Absorption-line redshifts for 37 galaxies in 31 remote clusters have been measured with the new sky-subtracting prism spectrograph, using an SIT television detector with digital readout. Twenty-five galaxies in the sample have redshifts larger than z = 0.20. New photoelectric BVR photometry has been obtained for 16 of the clusters. Magnitudes corrected for aperture effect, K-dimming, galactic absorption, cluster richness, and Bautz-Morgan contrast effect are listed. The data permit an improvement in the statistics of the Hubble diagram to z = 0.28 and a new determination of the dependence of the absolute magnitude of the brightest cluster galaxy on cluster richness. A small correlation with richness is found whose amplitude is 0.20 ± 0.18 mag over all Abell richness classes 0-4. As in previous studies, the shallowness of the correlation requires that if M(l) is governed by a general luminosity function, Φ(M), for fainter members, then the slope of Φ(M) at M(I) must be very steep. The 16 new clusters with photometry have a dispersion in M(l) of 0.26 mag. The dispersion of fully corrected magnitudes for the 65 clusters in the total sample is 0.276 mag. The new points in the Hubble diagram give no indication that the (m, z) relation deviates from the q_0(formal) = + 1 line (i.e. with no evolutionary correction applied), nor that 3C 295 is abnormally bright. The need for more data with z > 0.3 is stressed.