HeCS-red: Dense Hectospec Surveys of redMaPPer-selected Clusters

Abstract

Abstract We use dense redshift surveys to explore the properties of galaxy clusters selected from the redMaPPer catalog. Our new survey, Hectospec Cluster Survey of red-sequence-selected clusters (HeCS-red), includes 10,589 new or remeasured redshifts from MMT/Hectospec observations of 27 redMaPPer clusters at redshifts z = 0.08–0.25 with large estimated richnesses (richness estimate λ > 64). Our spectra confirm that each candidate cluster corresponds to a large overdensity in redshift space. The redMaPPer photometric redshifts have a slight bias toward higher redshifts. We measure the relation between velocity dispersion σ p and λ. The relation shows intrinsic scatter of 24% (and a range of 2x) in velocity dispersion at fixed richness. We extend our analysis to HeCS-red-ext, a sample that includes several clusters selected by X-ray flux or Sunyaev–Zeldovich signal. The heterogeneous sample of 121 clusters in HeCS-red-ext shows similar intrinsic scatter in the σ p –λ relation as the HeCS-red sample, but the range of σ p at fixed richness increases to 3x. We evaluate the membership probability estimates P mem for individual galaxies provided by redMaPPer. The spectroscopic membership fraction is larger than P mem for 0.05 ≤ P mem ≤ 0.7; conversely, the spectroscopic membership fraction is smaller than P mem at P mem ≥ 0.8. We compare spectroscopic richness estimates with redMaPPer richness estimates and find good agreement on average, but a range of a factor of two in spectroscopic richness at fixed redMaPPer richness. Overall, within the high-richness and low-redshift cut of our sample, spectroscopically estimated parameters such as velocity dispersion correlate well with photometric richness estimates, although the relations contain substantial scatter.