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Published September 2007 | Published
Journal Article Open

Weak Gravitational Lensing with COSMOS: Galaxy Selection and Shape Measurements


With a primary goal of conducting precision weak-lensing measurements from space, the COSMOS survey has imaged the largest contiguous area observed by Hubble Space Telescope to date, using the Advanced Camera for Surveys (ACS). This is the first paper in a series in which we describe our strategy for addressing the various technical challenges in the production of weak-lensing measurements from COSMOS data. We first construct a source catalog from 575 ACS/WFC tiles (1.64 deg^2) subsampled at a pixel scale of 0.03". Defects and diffraction spikes are carefully removed, leaving a total of 1.2 × 10^6 objects to a limiting magnitude of F814W = 26.5. This catalog is made publicly available. Multiwavelength follow-up observations of the COSMOS field provide photometric redshifts for 73% of the source galaxies in the lensing catalog. We analyze and discuss the COSMOS redshift distribution and show broad agreement with other surveys to z ~ 1. Our next step is to measure the shapes of galaxies and correct them for the distortion induced by the time-varying ACS point-spread function and for charge transfer efficiency (CTE) effects. Simulated images are used to derive the shear susceptibility factors that are necessary in transforming shape measurements into unbiased shear estimators. For every galaxy we derive a shape measurement error and utilize this quantity to extract the intrinsic shape noise of the galaxy sample. Interestingly, our results indicate that intrinsic shape noise varies little with size, magnitude, or redshift. Representing a number density of 66 galaxies per arcmin^2, the final COSMOS weak-lensing catalog contains 3.9 × 10^5 galaxies with accurate shape measurements. The properties of the COSMOS weak-lensing catalog described throughout this paper will provide key input numbers for the preparation and design of next-generation wide field space missions.

Additional Information

© 2007 The American Astronomical Society. Received 2006 September 21; accepted 2007 February 11. The HST COSMOS Treasury program was supported through NASA grant HST-GO-09822. We wish to thank our referee for useful comments and Kevin Bundy for carefully reading the manuscript. We also thank Tony Roman, Denise Taylor, and David Soderblom for their assistance in planning and scheduling of the extensive COSMOS observations. We gratefully acknowledge the contributions of the entire COSMOS collaboration, consisting of more than 70 scientists. More information on the COSMOS survey is available at http://www.astro.caltech.edu/~cosmos . It is a pleasure to acknowledge the excellent services provided by the NASA IPAC/IRSA staff (Anastasia Laity, Anastasia Alexov, Bruce Berriman, and John Good) in providing online archive and server capabilities for the COSMOS data sets. The COSMOS Science meeting in 2005 May was supported in part by the NSF through grant OISE-0456439. C. H. is supported by a CITA National fellowship and, along with L. V. W., acknowledges support from NSERC and CIAR. In France, the COSMOS project is supported by CNES and the Programme National de Cosmologie. J. P. K. acknowledges support from CNRS.

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August 22, 2023
October 19, 2023