Weak lensing using only galaxy position angles

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Abstract

We develop a method for performing a weak lensing analysis using only measurements of galaxy position angles. By analysing the statistical properties of the galaxy orientations given a known intrinsic ellipticity distribution, we show that it is possible to obtain estimates of the shear by minimizing a χ2 statistic. The method is demonstrated using simulations where the components of the intrinsic ellipticity are taken to be Gaussian distributed. Uncertainties in the position angle measurements introduce a bias into the shear estimates which can be reduced to negligible levels by introducing a correction term into the formalism. We generalize our approach by developing an algorithm to obtain direct shear estimators given any azimuthally symmetric intrinsic ellipticity distribution. We introduce a method of measuring the position angles of the galaxies from noisy pixelized images, and propose a method to correct for biases which arise due to pixelization and correlations between measurement errors and galaxy ellipticities. We also develop a method to constrain the sample of galaxies used to obtain an estimate of the intrinsic ellipticity distribution such that fractional biases in the resulting shear estimates are below a given threshold value. We demonstrate the angle-only method by applying it to simulations where the ellipticities are taken to follow a lognormal distribution. We compare the performance of the position-angle-only method with the standard method based on full ellipticity measurements by reconstructing lensing convergence maps from both numerical simulations and from the Canada-France-Hawaii Lensing Survey data. We find that the difference between the convergence maps reconstructed using the two methods is consistent with noise.

Author

Whittaker, Lee; Brown, Michael L.; Battye, Richard A.

Journal

Monthly Notices of the Royal Astronomical Society

Paper Publication Date

December 2014

Paper Type

Astrostatistics