Clustering of red galaxies near the radio-loud quasar 1335.8+2834 at z = 1.1

We have obtained new deep optical and near-infrared images of the field of the radio-loud quasar 1335.8+2834 at z = 1.086, where an excess in the surface number density of galaxies was reported by Hutchings et al. from optical data. We found a significant clustering of objects with very red optical-near-infrared colors, 4 ≲ R - K ≲ 6 and 3 ≲ I - K ≲ 5, near the quasar. The colors and magnitudes of the reddest objects are consistent with those of old (12 Gyr old at z = 0) passively evolving elliptical galaxies seen at z = 1.1, clearly defining a "red envelope" like that found in galaxy clusters at similar or lower redshifts. This evidence strongly suggests that the quasar resides in a moderately rich cluster of galaxies (richness class ≥0). There is also a relatively large fraction of objects with moderately red colors (3.5 < R - K < 4.5) that have a distribution on the sky similar to that of the reddest objects. They may be interpreted as cluster galaxies with some recent or ongoing star formation.