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Problems using ratios of galaxy shape moments in requirements for weak lensing surveys

Israel, H.; Kitching, T.D.; Massey, R.; Cropper, M.

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Authors

H. Israel

T.D. Kitching

M. Cropper



Abstract

Context. The shapes of galaxies are typically quantified by ratios of their quadrupole moments. Knowledge of these ratios (i.e. their measured standard deviation) is commonly used to assess the efficiency of weak gravitational lensing surveys. For faint galaxies, observational noise can make the denominator close to zero, so the ratios become ill-defined. Aims. Since the requirements cannot be formally tested for faint galaxies, we explore two complementary mitigation strategies. In many weak lensing contexts, the most problematic sources can be removed by a cut in measured size. This first technique is applied frequently. As our second strategy, we propose requirements directly on the quadrupole moments rather than their ratio. Methods. As an example of the first strategy, we have investigated how a size cut affects the required precision of the charge transfer inefficiency model for two shape measurement settings. For the second strategy, we analysed the joint likelihood distribution of the image quadrupole moments measured from simulated galaxies, and propagate their (correlated) uncertainties into ellipticities. Results. Using a size cut, we find slightly wider tolerance margins for the charge transfer inefficiency parameters compared to the full size distribution. However, subtle biases in the data analysis chain may be introduced. These can be avoided using the second strategy. To optimally exploit a Stage-IV dark energy survey, we find that the mean and standard deviation of a population of galaxies’ quadrupole moments must to be known to better than 1.4 × 10-3 arcsec2, or the Stokes parameters to 1.9 × 10-3 arcsec2. Conclusions. Cuts in measured size remove sources that otherwise make ellipticity statistics of weak lensing galaxy samples diverge. However, size cuts bias the source population non-trivially. Assessing weak lensing data quality directly on the quadrupole moments instead mitigates the need for size cuts. Such testable requirements can form the basis for performance validation of future instruments.

Citation

Israel, H., Kitching, T., Massey, R., & Cropper, M. (2017). Problems using ratios of galaxy shape moments in requirements for weak lensing surveys. Astronomy & Astrophysics, 598, Article A46. https://doi.org/10.1051/0004-6361/201628394

Journal Article Type Article
Acceptance Date Sep 22, 2016
Online Publication Date Jan 30, 2017
Publication Date Jan 30, 2017
Deposit Date Mar 13, 2017
Publicly Available Date Mar 17, 2017
Journal Astronomy and astrophysics.
Print ISSN 0004-6361
Electronic ISSN 1432-0746
Publisher EDP Sciences
Peer Reviewed Peer Reviewed
Volume 598
Article Number A46
DOI https://doi.org/10.1051/0004-6361/201628394
Public URL https://durham-repository.worktribe.com/output/1361434

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Copyright Statement
Reproduced with permission from Astronomy & Astrophysics, © ESO






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