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Abstract: Accurate photometric redshifts are among the key requirements for precisionweak lensing measurements. Both the large size of the Sloan Digital Sky SurveySDSS and the existence of large spectroscopic redshift samples that areflux-limited beyond its depth have made it the optimal data source fordeveloping methods to properly calibrate photometric redshifts for lensing.Here, we focus on galaxy-galaxy lensing in a survey with spectroscopic lensredshifts, as in the SDSS. We develop statistics that quantify the effect ofsource redshift errors on the lensing calibration and on the weighting scheme,and show how they can be used in the presence of redshift failure and samplingvariance. We then demonstrate their use with 2838 source galaxies withspectroscopy from DEEP2 and zCOSMOS, evaluating several public photometricredshift algorithms, in two cases including a full pz for each object, andfind lensing calibration biases as low as 1% due to fortuitous cancellation oftwo types of bias or as high as 20% for methods in active use despite thesmall mean photoz bias of these algorithms. Our work demonstrates thatlensing-specific statistics must be used to reliably calibrate the lensingsignal, due to asymmetric effects of frequently non-Gaussian photoz errors.We also demonstrate that large-scale structure LSS can strongly impact thephotoz calibration and its error estimation, due to a correlation between theLSS and the photoz errors, and argue that at least two independent degree-scalespectroscopic samples are needed to suppress its effects. Given the size of ourspectroscopic sample, we can reduce the galaxy-galaxy lensing calibration errorwell below current SDSS statistical errors.

Autor: R. Mandelbaum, U. Seljak, C. M. Hirata, S. Bardelli, M. Bolzonella, A. Bongiorno, M. Carollo, T. Contini, C. E. Cunha, B. Garilli


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