Shedding light on the matter of Abell 781

• Published in: Monthly notices of the Royal Astronomical Society Vol. 437; no. 4; pp. 3578 - 3585
• Main Authors: Wittman, D., Dawson, William, Benson, Bryant
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01.02.2014
• Subjects: Calibration; Gravity; Red shift; Signal to noise ratio; Stars & galaxies; X-ray astronomy; gravitational lensing: weak; methods: statistical; galaxies: clusters: individual: Abell 781
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stt2151

The galaxy cluster Abell 781 West 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.