| Summary: | We use weak gravitational lensing to measure the masses of five galaxy clusters selected from the South Pole Telescope (SPT) survey, with the primary goal of comparing these with the SPT Sunyaev-Zel'dovich (SZ) and X-ray-based mass estimates. The clusters span redshifts 0.28 < z < 0.43 and have masses M_(500) > 2 × 10^(14) h^(–1) M_☉, and three of the five clusters were discovered by the SPT survey. We observed the clusters in the g'r'i' passbands with the Megacam imager on the Magellan Clay 6.5 m telescope. We measure a mean ratio of weak-lensing (WL) aperture masses to inferred aperture masses from the SZ data, both within an aperture of R_(500,SZ) derived from the SZ mass, of 1.04 ± 0.18. We measure a mean ratio of spherical WL masses evaluated at R_(500,SZ) to spherical SZ masses of 1.07 ± 0.18, and a mean ratio of spherical WL masses evaluated at R_(500,WL) to spherical SZ masses of 1.10 ± 0.24. We explore potential sources of systematic error in the mass comparisons and conclude that all are subdominant to the statistical uncertainty, with dominant terms being cluster concentration uncertainty and N-body simulation calibration bias. Expanding the sample of SPT clusters with WL observations has the potential to significantly improve the SPT cluster mass calibration and the resulting cosmological constraints from the SPT cluster survey. These are the first WL detections using Megacam on the Magellan Clay telescope. © 2012 American Astronomical Society. Received 2012 May 11; accepted 2012 August 26; published 2012 September 26. We thank M. Holman for trading Megacam observing time for a proof-of-concept study, which enabled the first ever detection of weak gravitational shear using the Clay-Megacam system (SPT-CL J0516-5430). We extend thanks to P. Protopapas for observing assistance, as well as the entire staff of Las Campanas Observatory and the Megacam instrument scientists. We gratefully acknowledge Risa Wechsler, Michael Busha, and Matt Becker for providing us simulated shear ...
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