Exploring the contamination of the DES-Y1 cluster sample with SPT-SZ selected clusters

Abstract

We perform a cross validation of the cluster catalog selected by the red-sequence Matched-filter Probabilistic Percolation algorithm (redMaPPer) in Dark Energy Survey year 1 (DES-Y1) data by matching it with the Sunyaev-Zel’dovich effect (SZE) selected cluster catalog from the South Pole Telescope SPT-SZ survey. Of the 1005 redMaPPer selected clusters with measured richness $\hat{\lambda }>40$ in the joint footprint, 207 are confirmed by SPT-SZ. Using the mass information from the SZE signal, we calibrate the richness–mass relation using a Bayesian cluster population model. We find a mass trend λ∝MB consistent with a linear relation (B ∼ 1), no significant redshift evolution and an intrinsic scatter in richness of σλ = 0.22 ± 0.06. By considering two error models, we explore the impact of projection effects on the richness–mass modelling, confirming that such effects are not detectable at the current level of systematic uncertainties. At low richness SPT-SZ confirms fewer redMaPPer clusters than expected. We interpret this richness dependent deficit in confirmed systems as due to the increased presence at low richness of low mass objects not correctly accounted for by our richness-mass scatter model, which we call contaminants. At a richness $\hat{\lambda }=40$, this population makes up $>12\%$ (97.5 percentile) of the total population. Extrapolating this to a measured richness $\hat{\lambda }=20$ yields $>22\%$ (97.5 percentile). With these contamination fractions, the predicted redMaPPer number counts in different plausible cosmologies are compatible with the measured abundance. The presence of such a population is also a plausible explanation for the different mass trends (B ∼ 0.75) obtained from mass calibration using purely optically selected clusters. The mean mass from stacked weak lensing (WL) measurements suggests that these low mass contaminants are galaxy groups with masses ∼3- 5 × 1013 M⊙ which are beyond the sensitivity of current SZE and X-ray surveys but a natural target for SPT-3G and eROSITA.

Optical selection bias and projection effects in stacked galaxy cluster weak lensing

“…An alternative approach is to model the correlated residuals between observables and fit for the correlation coefficients (e.g. Grandis et al 2021a;Chiu et al 2021). This approach extends the analytic model described in Section 2 and can be part of the forwardmodelling procedure.…”

“…In addition, the cross comparison between optical and SZ clusters can be used to study projection effects (e.g. Grandis et al 2021a); for example, a cluster heavily contaminated by galaxies along the line of sight would have a lower SZ signal than expected from its richness.…”

The effect of selection -- a tale of cluster mass measurement bias induced by correlation and projection

“…These effects may cause additional correlation between the cluster's observable and its weak-lensing mass measurement, and thus we can consider alleviating their effects through modeling the correlations ( as in, e.g., Zhang et al 2019;Grandis et al 2020Grandis et al , 2021b. In this paper, we demonstrate the potential effectiveness of this correlation approach by examining the projected observables of clustersized dark matter halos and their correlations with weak-lensing like mass measurements in simulation.…”

Snowmass2021: Opportunities from Cross-survey Analyses of Static Probes

“…For instance, optical imaging can provide precise weak lensing mass calibration of SZselected clusters [109,110]. SZ observations, on the other hand, can be used to improve the robustness of optical cluster selection [111,112].…”