Potential scientific synergies in weak lensing studies between the CSST andEuclidspace probes
Authors: Liu D. Z. , Meng X. M. , Er X. Z. , Fan Z. H. , Kilbinger M. , Li G. L. , Li R. , Schrabback T. , Scognamiglio D. , Shan H. Y. , Tao C. , Ting Y. S. , Zhang J. , Cheng S. H. , Farrens S. , Fu L. P. , Hildebrandt H. , Kang X. , Kneib J. P. , Liu X. K. , Mellier Y. , Nakajima R. , Schneider P. , Starck J. L. , Wei C. L. , Wright A. H. , Zhan H. 
Published: November 2022

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:Euclidand 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, ybands from CSST with the VIS,Y, J, Hbands fromEuclid, and other optical bands from the ground-basedVera C. RubinObservatory 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. ForEuclid, 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 forEuclidto obtain more accurate PSF measurements and to calibrate the color and color-gradient biases for galaxy shear measurements.

Methods.We created image simulations, using theHubbleDeep UV data as the input catalog, for different surveys and quantified the photo-zperformance using theEAZYtemplate fitting code. For the blending analyses, we employed high-resolution HST-ACS CANDELSF606WandF814Wdata to synthesize mock simulated data forEuclid, 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 forEuclid.

Results.The sky coverage ofEuclidlies entirely within the CSST footprint. The combination ofEuclidwith the CSST data can thus be done more uniformly than with the various ground-based data that are part of theEuclidsurvey. Our studies show that by combiningEuclidand CSST, we can reach a photo-zprecision ofσNMAD ≈ 0.04 and an outlier fraction ofη ≈ 2.4% at the nominal depth of theEuclidWide Survey (VIS < 24.5 AB mag). For CSST, including theEuclidY, J, Hbands reduces the overall photo-zoutlier fraction from ∼8.5% to 2.4%. Forz > 1, the improvements are even more significant. Because of the similarly high resolutions, the data combination ofEuclidand 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 forEuclidVIS shear measurements. The color-gradient bias forEuclidgalaxies 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.