Potential scientific synergies in weak lensing studies between the CSST and Euclid space probes

¹ South-Western Insitiute for Astronomy Research, Yunnan University, Kunming 650500, PR China
e-mail: zuhuifan@ynu.edu.cn
² Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
³ AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, 91191 Gif-sur-Yvette, France
e-mail: martin.kilbinger@cea.fr
⁴ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, PR China
⁵ Zhejiang University-Purple Mountain Observatory Joint Research Center for Astronomy, Zhejiang University, Hangzhou 310027, PR China
⁶ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁷ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, PR China
⁸ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, PR China
⁹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁰ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
¹¹ Shanghai Astronomical Observatory (SHAO), Nandan Road 80, Shanghai 200030, PR China
¹² University of Chinese Academy of Sciences, Beijing 100049, PR China
¹³ CPPM, Aix-Marseille Université, CNRS/IN2P3, CPPM UMR 7346, 13288 Marseille, France
¹⁴ Research School of Astronomy & Astrophysics, Australian National University, Cotter Rd., Weston, ACT 2611, Australia
¹⁵ Research School of Computer Science, Australian National University, Acton, ACT 2601, Australia
¹⁶ Department of Astronomy, Shanghai Jiao Tong University, Shanghai 200240, PR China
¹⁷ Department of Physics and Astronomy, The Johns Hopkins University, 3400 N Charles Street, Baltimore, MD 21218, USA
¹⁸ Shanghai Key Lab for Astrophysics, Shanghai Normal University, Shanghai 200234, PR China
¹⁹ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), German Centre for Cosmological Lensing, 44780 Bochum, Germany
²⁰ Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
²¹ Aix-Marseille Univ., CNRS, CNES, LAM, 13388 Marseille, France
²² CEA Saclay, DFR/IRFU, Service d’Astrophysique, Bat. 709, 91191 Gif-sur-Yvette, France
²³ Institut d’Astrophysique de Paris, 98 Bis Boulevard Arago, 75014 Paris, France
²⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China

Received: 9 May 2022
Accepted: 28 October 2022

Abstract

Aims. With the next generation of large surveys poised to join the ranks of observational cosmology in the near future, it is important to explore their potential synergies and to maximize their scientific outcomes. In this study, we aim to investigate the complementarity of two upcoming space missions: Euclid and the China Space Station Telescope (CSST), both of which will be focused on weak gravitational lensing for cosmology. In particular, we analyze the photometric redshift (photo-z) measurements by combining NUV, u, g, r, i, z, y bands from CSST with the VIS, Y, J, H bands from Euclid, and other optical bands from the ground-based Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) and Dark Energy Survey. We also consider the advantages of combining the two space observational data in simplifying image deblending. For Euclid, weak lensing measurements use the broad optical wavelength range of 550−900 nm, for which chromatic point-spread function (PSF) effects are significant. For this purpose, the CSST narrow-band data in the optical can provide valuable information for Euclid to obtain more accurate PSF measurements and to calibrate the color and color-gradient biases for galaxy shear measurements.

Methods. We created image simulations, using the Hubble Deep UV data as the input catalog, for different surveys and quantified the photo-z performance using the EAZY template fitting code. For the blending analyses, we employed high-resolution HST-ACS CANDELS F606W and F814W data to synthesize mock simulated data for Euclid, CSST, and an LSST-like survey. We analyzed the blending fraction for different cases as well as the blending effects on galaxy photometric measurements. Furthermore, we demonstrated that CSST can provide a large enough number of high signal-to-noise ratio multi-band galaxy images to calibrate the color-gradient biases for Euclid.

Results. The sky coverage of Euclid lies entirely within the CSST footprint. The combination of Euclid with the CSST data can thus be done more uniformly than with the various ground-based data that are part of the Euclid survey. Our studies show that by combining Euclid and CSST, we can reach a photo-z precision of σ_NMAD ≈ 0.04 and an outlier fraction of η ≈ 2.4% at the nominal depth of the Euclid Wide Survey (VIS < 24.5 AB mag). For CSST, including the Euclid Y, J, H bands reduces the overall photo-z outlier fraction from ∼8.5% to 2.4%. For z > 1, the improvements are even more significant. Because of the similarly high resolutions, the data combination of Euclid and CSST can be relatively straightforward for photometry measurements. On the other hand, to include ground-based data, sophisticated deblending utilizing priors from high-resolution space observations are required. The multi-band data from CSST are very helpful in controlling the chromatic PSF effect for Euclid VIS shear measurements. The color-gradient bias for Euclid galaxies with different bulge-to-total flux ratio at different redshifts can be well calibrated to the level of 0.1% using galaxies from the CSST deep survey.