A Rich Cluster of Galaxies near the Quasar B2 1335+28 at z = 1.1: Color Distribution and Star Formation Properties^*

We previously reported a significant clustering of red galaxies (R-K = 3.5-6) around the radio-loud quasar B2 1335+28 at z = 1.086. In this paper, we establish the existence of a rich cluster at the quasar redshift and study the properties of the cluster galaxies through further detailed analysis of the photometric data. We also list the positions, K-band magnitudes, and colors of all K < 19 objects. The near-infrared (NIR) K-band imaging data presented in earlier paper, together with some additional K-band data, is newly analyzed to study the extent of the clustering of the red galaxies. We also constrain the cluster redshift by applying a robust photometric redshift estimator and find a strong peak around z = 1.1. The color distribution of the galaxies in the cluster is quite broad, and the fraction of blue galaxies (~70%) is much larger than in intermediate-redshift clusters. With the help of evolutionary synthesis models, we show that this color distribution can be explained by galaxies with various amounts of star formation activity mixed with the old stellar populations. Notably, there are about a dozen galaxies that show very red optical-NIR colors but at the same time show significant UV excess with respect to passive-evolution models. They can be interpreted as old early-type galaxies with a small amount (a few percent by mass) of star formation. The fact that the UV-excess red galaxies are more abundant than the quiescent red ones suggests that a large fraction of old galaxies in this cluster are still forming stars to some extent. However, a sequence of quiescent red galaxies is clearly identified on the R-K versus K color-magnitude (C-M) diagram. The slope and zero point of their C-M relation appear to be consistent with those expected for the precursors of the C-M relation of present-day cluster ellipticals when observed at z = 1.1. The scatter around the C-M line (~0.2 mag in R-K) is twice as large as that of the morphologically selected early-type galaxies observed in rich clusters at z < 1, although the uncertainty in the value of the scatter is quite large. We estimate that the Abell richness class of the cluster is R_Abell ~ 1. New X-ray data presented here place an upper limit of L_X < 2 × 10⁴⁴ ergs s^-1 for the cluster luminosity. Finally, we investigate the distribution of the galaxies over larger spatial scales using our optical images, which cover a much larger area than the near-infrared ones. We find evidence that the cluster is located within some lumpy overdense structures, suggesting that the whole system has not yet relaxed completely and is still dynamically young.