Bleem, L. E.Bocquet, S.Stalder, BGladders, Michael DAde, P. A. R.Allen, StevenAnderson, A. J.Annis, JamesAshby, M. L. N.Austermann, J. E.Lidman, Chris2023-03-212023-03-210067-0049http://hdl.handle.net/1885/287229We describe the observations and resultant galaxy cluster catalog from the 2770 deg2 SPTpol Extended Cluster Survey (SPT-ECS). Clusters are identified via the Sunyaev–Zel’dovich (SZ) effect and confirmed with a combination of archival and targeted follow-up data, making particular use of data from the Dark Energy Survey (DES). With incomplete follow-up we have confirmed as clusters 244 of 266 candidates at a detection significance ξ 5 and an additional 204 systems at 4 < ξ < 5. The confirmed sample has a median mass of ~ ´ M Mh - 500c 4.4 1014 70 1  and a median redshift of z = 0.49, and we have identified 44 strong gravitational lenses in the sample thus far. Radio data are used to characterize contamination to the SZ signal; the median contamination for confirmed clusters is predicted to be ∼1% of the SZ signal at the ξ > 4 threshold, and <4% of clusters have a predicted contamination >10% of their measured SZ flux. We associate SZ-selected clusters, from both SPT-ECS and the SPT-SZ survey, with clusters from the DES redMaPPer sample, and we find an offset distribution between the SZ center and central galaxy in general agreement with previous work, though with a larger fraction of clusters with significant offsets. Adopting a fixed Planck-like cosmology, we measure the optical richness–SZ mass (l - M) relation and find it to be 28% shallower than that from a weak-lensing analysis of the DES data—a difference significant at the 4σ level—with the relations intersecting at λ = 60. The SPT-ECS cluster sample will be particularly useful for studying the evolution of massive clusters and, in combination with DES lensing observations and the SPT-SZ cluster sample, will be an important component of future cosmological analysesThe Melbourne authors acknowledge support from the Australian Research Council’s Discovery Projects scheme (DP150103208).application/pdfen-AU© 2020. The American Astronomical SocietyGalaxy clustersStrong gravitational lensingLarge-scale structure of the universe202010.3847/1538-4365/ab69932022-01-16