ORCA : the overdense red-sequence cluster algorithm

Abstract

2 sample of the deep Sloan Digital Sky Survey (SDSS) equatorial stripe (Stripe 82), from which we detect 97 clusters with z≤ 0.6. Benefitting from deeper data, we are 100 per cent complete in the maxBCG optically selected cluster catalogue (based on shallower single-epoch SDSS data) and find an additional 78 previously unidentified clusters. The second data set is a mock Medium Deep Survey Pan-STARRS catalogue, based on the Λ cold dark matter (ΛCDM) model and a semi-analytic galaxy formation recipe. Knowledge of galaxy–halo memberships in the mock catalogue allows for the quantification of algorithm performance. We detect 305 mock clusters in haloes with mass >1013 h−1 M⊙ at z≲ 0.6 and determine a spurious detection rate of <1 per cent, consistent with tests on the Stripe 82 catalogue. The detector performs well in the recovery of model ΛCDM clusters. At the median redshift of the catalogue, the algorithm achieves >75 per cent completeness down to halo masses of 1013.4 h−1 M⊙ and recovers >75 per cent of the total stellar mass of clusters in haloes down to 1013.8 h−1 M⊙. A companion paper presents the complete cluster catalogue over the full 270-deg2 Stripe 82 catalogue.