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Euclid preparation
IV. Impact of undetected galaxies on weak-lensing shear measurements

Euclid Collaboration, N. Martinet, T. Schrabback, H. Hoekstra, M. Tewes, R. Herbonnet, P. Schneider, B. Hernandez-Martin, A. N. Taylor, J. Brinchmann, C. S. Carvalho, M. Castellano, G. Congedo, B. R. Gillis, E. Jullo, M. Kümmel, S. Ligori, P. B. Lilje, C. Padilla, D. Paris, J. Peacock, S. Pilo, A. Pujol, D. Scott, R. Toledo-Moreo

In modern weak-lensing surveys, the common approach to correct for residual systematic biases in the shear is to calibrate shape measurement algorithms using simulations. These simulations must fully capture the complexity of the observations to avoid introducing any additional bias. In this paper we study the importance of faint galaxies below the observational detection limit of a survey. We simulate simplified Euclid VIS images including and excluding this faint population, and measure the shift in the multiplicative shear bias between the two sets of simulations. We measure the shear with three different algorithms: a moment-based approach, model fitting, and machine learning. We find that for all methods, a spatially uniform random distribution of faint galaxies introduces a shear multiplicative bias of the order of a few times 10−3. This value increases to the order of 10−2 when including the clustering of the faint galaxies, as measured in the Hubble Space Telescope Ultra-Deep Field. The magnification of the faint background galaxies due to the brighter galaxies along the line of sight is found to have a negligible impact on the multiplicative bias. We conclude that the undetected galaxies must be included in the calibration simulations with proper clustering properties down to magnitude 28 in order to reach a residual uncertainty on the multiplicative shear bias calibration of a few times 10−4, in line with the 2 × 10−3 total accuracy budget required by the scientific objectives of the Euclid survey. We propose two complementary methods for including faint galaxy clustering in the calibration simulations.

gravitational lensing: weak / cosmology: observationsxs / surveys

Based on Hubble Space Telescope Ultra-Deep Field (HST-UDF) data.

Astronomy & Astrophysics
Volume 627, Article Number A59, Number of pages 20
2019 July

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia