A measurement of the mean central optical depth of galaxy clusters via the pairwise kinematic Sunyaev-Zel'dovich effect with SPT-3G and DES

We infer the mean optical depth of a sample of optically-selected galaxy clusters from the Dark Energy Survey (DES) via the pairwise kinematic Sunyaev-Zel’dovich (kSZ) effect. The pairwise kSZ signal between pairs of clusters drawn from the DES Year-3 cluster catalog is detected at 4 . 1 σ in cosmic microwave background (CMB) temperature maps from two years of observations with the SPT-3G camera on the South Pole Telescope. After cuts, there are 24,580 clusters in the ∼ 1,400 deg 2 of the southern sky observed by both experiments. We infer the mean optical depth of the cluster sample with two techniques. The optical depth inferred from the pairwise kSZ signal is ¯ τ e = (2 . 97 ± 0 . 73) × 10 − 3 , while that inferred from the thermal SZ signal is ¯ τ e = (2 . 51 ± 0 . 55) × 10 − 3 . The two measures agree at 0.6 σ . We perform a suite of systematic checks to test the robustness of the analysis.