Cosmology With Optically Selected Galaxy Clusters: Looking Forward to DES

Eli Rykoff


In the next few years, large scale optical surveys such as DES will detect galaxy clusters as high as z~1 over several thousand square degrees. In conjunction with stacked weak-lensing calibration, DES optical clusters will be used to place tight constraints on cosmological parameters, extending successful work at low redshift with SDSS data. Unfortunately, cluster mass is not a direct observable, and we must rely on other quantities that trace mass. Optical richness based on multi-band photometric data is one such mass proxy, although it may suffer from large scatter in the mass-richness relation. Minimizing this scatter, and understanding the covariance among the various cluster observables, is necessary to achieve our goals of precision cosmology. To this end, we have made use of the wealth of data available for galaxy clusters by combining large samples of clusters with weak-lensing data and X-ray photon maps to develop an optimized matched filter richness estimator that minimizes mass-richness scatter. This richness estimator forms the basis of redMaPPer, a new red sequence optical cluster finder for use on multi-band photometric data such as DES. Extensive testing on both simulated mock DES catalogs and real SDSS data have shown that we achieve very precise redshift calibration, and testing on the mocks have shown high purity and completeness as well as low mass scatter up to z~1.0. By leveraging faint red cluster members we can also calibrate photometric redshifts for red galaxies to much fainter magnitudes than is typically possible with spectroscopic samples, improving large-scale structure constraints available from photometric data.