Optical and Sunyaev–Zel'dovich observations of a new sample of distant rich galaxy clusters in the ROSAT All Sky

Finding a sample of the most massive clusters with redshifts z > 0.6 can provide an interesting consistency check of the Λ cold dark matter (ΛCDM) model. Here, we present results from our search for clusters with 0.6 z 1.0 where the initial candidates were selected by cross-correlating the ROSAT All Sky Survey (RASS) faint and bright source catalogues with red galaxies from the Sloan Digital Sky Survey DR8. Our survey thus covers 10 000 deg2, much larger than previous studies of this kind. Deeper follow-up observations in three bands using the William Herschel Telescope and the Large Binocular Telescope were performed to confirm the candidates, resulting in a sample of 44 clusters for which we present richnesses and red sequence redshifts, as well as spectroscopic redshifts for a subset. At least two of the clusters in our sample are comparable in richness to RCS2-J232727.7−020437, one of the richest systems discovered to date. We also obtained new observations with the Combined Array for Research in Millimeter Astronomy for a subsample of 21 clusters. For 11 of those we detect the Sunyaev–Zel'dovich effect signature. The Sunyaev–Zel'dovich signal allows us to estimate M200 and check for tension with the cosmological standard model. We find no tension between our cluster masses and the ΛCDM model.