Constraints on the richness–mass relation and the optical-SZE positional offset distribution for SZE-selected clusters

Abstract

In this study, we cross-match galaxy cluster candidates selected via their Sunyaev–Zel'dovich effect (SZE) signatures in 129.1 deg<sup>2</sup> of the South Pole Telescope 2500d SPT-SZ survey with optically identified clusters selected from the Dark Energy Survey science verification data. We identify 25 clusters between 0.1 ≲ z ≲ 0.8 in the union of the SPT-SZ and redMaPPer (RM) samples. RM is an optical cluster finding algorithm that also returns a richness estimate for each cluster. We model the richness λ-mass relation with the following function <ln λ|M<sub>500</sub>> ∝ B<sub>λ</sub>ln M<sub>500</sub> + C<sub>λ</sub>ln E(z) and use SPT-SZ cluster masses and RM richnesses λ to constrain the parameters. We find B<sub>λ</sub>=1.14<sup>+0.21</sup><sub>–0.18</sub> and Cλ=0.73<sup>+0.77</sup><sub>–0.75</sub>. The associated scatter in mass at fixed richness is σ<sub>lnM|λ</sub> = 0.18<sup>+0.08</sup><sub>–0.05</sub> at a characteristic richness λ = 70. We demonstrate that our model provides an adequate description of the matched sample, showing that the fraction of SPT-SZ-selected clusters with RM counterparts is consistent with expectations and that the fraction of RM-selected clusters with SPT-SZ counterparts is in mild tension with expectation. We model the optical-SZE cluster positional offset distribution with the sum of two Gaussians, showing that it is consistent with a dominant, centrally peaked population and a subdominant population characterized by larger offsets. We also cross-match the RM catalogue with SPT-SZ candidates below the official catalogue threshold significance ξ = 4.5, using the RM catalogue to provide optical confirmation and redshifts for 15 additional clusters with ξ ε [4, 4.5].