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Weighing the Giants – II. Improved calibration of photometry from stellar colours and accurate photometric redshifts

Kelly, Patrick L. and von der Linden, Anja and Applegate, Douglas E. and Allen, Mark T. and Allen, Steven W. and Burchat, Patricia R. and Burke, David L. and Ebeling, Harald and Capak, Peter and Czoske, Oliver and Donovan, David and Mantz, Adam and Morris, R. Glenn (2014) Weighing the Giants – II. Improved calibration of photometry from stellar colours and accurate photometric redshifts. Monthly Notices of the Royal Astronomical Society, 439 (1). pp. 28-47. ISSN 0035-8711.

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We present improved methods for using stars found in astronomical exposures to calibrate both star and galaxy colours as well as to adjust the instrument flat-field. By developing a spectroscopic model for the Sloan Digital Sky Survey (SDSS) stellar locus in colour–colour space, synthesizing an expected stellar locus, and simultaneously solving for all unknown zero-points when fitting to the instrumental locus, we increase the calibration accuracy of stellar locus matching. We also use a new combined technique to estimate improved flat-field models for the Subaru SuprimeCam camera, forming ‘star flats’ based on the magnitudes of stars observed in multiple positions or through comparison with available measurements in the SDSS catalogue. These techniques yield galaxy magnitudes with reliable colour calibration (≲0.01–0.02 mag accuracy) that enable us to estimate photometric redshift probability distributions without spectroscopic training samples. We test the accuracy of our photometric redshifts using spectroscopic redshifts z_s for ∼5000 galaxies in 27cluster fields with at least five bands of photometry, as well as galaxies in the Cosmic Evolution Survey (COSMOS) field, finding σ((z_p − z_s)/(1 + z_s)) ≈ 0.03 for the most probable redshift z_p. We show that the full posterior probability distributions for the redshifts of galaxies with five-band photometry exhibit good agreement with redshifts estimated from thirty-band photometry in the COSMOS field. The growth of shear with increasing distance behind each galaxy cluster shows the expected redshift–distance relation for a flat Λ cold dark matter (Λ-CDM) cosmology. Photometric redshifts and calibrated colours are used in subsequent papers to measure the masses of 51 galaxy clusters from their weak gravitational shear and determine improved cosmological constraints. We make our python code for stellar locus matching publicly available at; the code requires only input catalogues and filter transmission functions.

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Additional Information:© 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2013 October 8. Received 2013 October 7; in original form 2013 August 1. First published online: February 4, 2014. We thank F. William High, Jorg Dietrich and Hendrik Hildebrandt for their expert advice and assistance on photometric calibration and redshift estimation. We also appreciate helpful discussions with Thomas Erben, C-J Ma, Daniel Coe and Carlos Cunha. This work is supported in part by the US Department of Energy under contract number DE-AC02-76SF00515. This work was also supported by the National Science Foundation under Grant nos. AST-0807458 and AST-1140019. MTA and PRB acknowledge the support of NSF grant PHY-0969487. AM acknowledges the support of NSF grant AST-0838187. The authors acknowledge support from programmes HST-AR-12654.01-A, HST-GO-12009.02- A and HST-GO-11100.02-A provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This work is also supported by the National Aeronautics and Space Administration through Chandra Award Numbers TM1-12010X, GO0-11149X, GO9-0141X and GO8-9119X issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. DEA recognizes the support of a Hewlett Foundation Stanford Graduate Fellowship. Based in part on data collected at Subaru Telescope (University of Tokyo) and obtained from the SMOKA, which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan. Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the CFHT, which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France and the University of Hawaii. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation and the US Department of Energy Office of Science. The SDSS-III website is This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, Caltech, under contract with NASA.
Funding AgencyGrant Number
Department of Energy (DOE)DE-AC02-76SF00515
NASAHST-GO-12009.02- A
NASANAS 5-26555
NASA Chandra AwardTM1-12010X
NASA Chandra AwardGO0-11149X
NASA Chandra AwardGO9-0141X
NASA Chandra AwardGO8-9119X
Hewlett Foundation Stanford Graduate FellowshipUNSPECIFIED
Subject Keywords:gravitational lensing: weak; methods: observational; techniques: photometric
Record Number:CaltechAUTHORS:20140428-102724638
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Official Citation:Kelly, P. L., von der Linden, A., Applegate, D. E., Allen, M. T., Allen, S. W., Burchat, P. R., . . . Morris, R. G. (2014). Weighing the Giants – II. Improved calibration of photometry from stellar colours and accurate photometric redshifts. Monthly Notices of the Royal Astronomical Society, 439(1), 28-47. doi: 10.1093/mnras/stt1946
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:45244
Deposited By: Jason Perez
Deposited On:28 Apr 2014 20:42
Last Modified:06 Jul 2015 17:31

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