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

Abstract

We use dense redshift surveys to explore the properties of galaxy clusters selected from the redMaPPer catalog of overdensities of red galaxies. Our new survey, HeCS-red (Hectospec Cluster Survey of red-sequence selected clusters), includes 10,589 new or remeasured redshifts from MMT/Hectospec observations of redMaPPer clusters at redshifts $z$=0.08-0.25 with large estimated richnesses (richness estimate $\lambda >64$). Our spectra confirm that each of these candidate clusters corresponds to an overdensity in redshift space. The redMaPPer photometric redshifts have a slight bias towards higher redshifts. We measure the scaling relation between velocity dispersion ${\sigma }_p$ and redMaPPer richness estimates $\lambda$. The observed relation shows intrinsic scatter of 24\% in velocity dispersion at fixed richness, and a range of a factor of two in measured ${\sigma }_p$ at fixed richness. We extend our analysis to HeCS-red-ext, a sample that includes several clusters selected by X-ray flux or SZ signal. The heterogeneous sample of 121 clusters in HeCS-red-ext shows similar intrinsic scatter, but the range of ${\sigma }_p$ at fixed richness increases to a factor of three. We evaluate the membership probability estimates $P_{mem}$ for individual galaxies provided by redMaPPer. The spectroscopic membership fraction is larger than $P_{mem}$ for $0.05\le P_{mem}\le 0.7$; conversely, it is smaller than $P_{mem}$ at $P_{mem}\ge 0.8$. We compare spectroscopic richness estimates to 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.

15 pages, 16 figures, 10,589 redshifts, submitted to ApJ, comments welcome

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