Title: Shape measurement biases from underfitting and ellipticity gradients

Abstract

With this study, precision weak gravitational lensing experiments require measurements of galaxy shapes accurate to <1 part in 1000. We investigate measurement biases, noted by Voigt and Bridle (2009) and Melchior et al. (2009), that are common to shape measurement methodologies that rely upon fitting elliptical-isophote galaxy models to observed data. The first bias arises when the true galaxy shapes do not match the models being fit. We show that this "underfitting bias" is due, at root, to these methods' attempts to use information at high spatial frequencies that has been destroyed by the convolution with the point-spread function (PSF) and/or by sampling. We propose a new shape-measurement technique that is explicitly confined to observable regions of k-space. A second bias arises for galaxies whose ellipticity varies with radius. For most shape-measurement methods, such galaxies are subject to "ellipticity gradient bias". We show how to reduce such biases by factors of 20–100 within the new shape-measurement method. The resulting shear estimator has multiplicative errors < 1 part in 10³ for high-S/N images, even for highly asymmetric galaxies. Without any training or recalibration, the new method obtains Q = 3000 in the GREAT08 Challenge of blind shear reconstruction on low-noise galaxies,more » several times better than any previous method.« less

Authors:

Bernstein, Gary M. ^[1]

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+ Show Author Affiliations

1. Univ. of Pennsylvania, Philadelphia, PA (United States)

Publication Date:

Sat Aug 21 00:00:00 EDT 2010

Research Org.:

Univ. of Pennsylvania, Philadelphia, PA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), High Energy Physics (HEP)

OSTI Identifier:

1203556

Grant/Contract Number:  

FG02-95ER40893

Resource Type:

Accepted Manuscript

Journal Name:

Monthly Notices of the Royal Astronomical Society

Additional Journal Information:

Journal Volume: 406; Journal Issue: 4; Journal ID: ISSN 0035-8711

Publisher:

Royal Astronomical Society

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS; measurement biases; gravitational lensing; data analysis