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Shedding light on the matter of Abell 781


The galaxy cluster Abell 781West has been viewed as a challenge to weak gravitational lensing mass calibration, as Cook & dell'Antonio found that the weak lensing signal-to-noise ratio in three independent sets of observations was consistently lower than expected from mass models based on X-ray and dynamical measurements. We correct some errors in statistical inference in Cook & dell'Antonio and show that their own results agree well with the dynamical mass and exhibit at most 2.2-2.9σ low compared to the X-ray mass, similar to the tension between the dynamical and X-ray masses. Replacing their simple magnitude cut with weights based on source photometric redshifts eliminates the tension between lensing and X-ray masses; in this case the weak lensing mass estimate is actually higher than, but still in agreement with, the dynamical estimate. A comparison of lensing analyses with and without photometric redshifts shows that a 1-2σ chance alignment of low-redshift sources lowers the signal-to-noise ratio observed by all previous studies which used magnitude cuts rather than photometric redshifts. The fluctuation is unexceptional, but appeared to be highly significant in Cook & dell'Antonio due to the errors in statistical interpretation. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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