Potential scientific synergies in weak lensing studies between the CSST and Euclid space probes
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 (photoz) 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 groundbased 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 pointspread function (PSF) effects are significant. For this purpose, the CSST narrowband data in the optical can provide valuable information for Euclid to obtain more accurate PSF measurements and to calibrate the color and colorgradient 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 photoz performance using the EAZY template fitting code. For the blending analyses, we employed highresolution HSTACS CANDELS F606W and F814W data to synthesize mock simulated data for Euclid, CSST, and an LSSTlike 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 signaltonoise ratio multiband galaxy images to calibrate the colorgradient 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 groundbased data that are part of the Euclid survey. Our studies show that by combining Euclid and CSST, we can reach a photoz 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 photoz 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 groundbased data, sophisticated deblending utilizing priors from highresolution space observations are required. The multiband data from CSST are very helpful in controlling the chromatic PSF effect for Euclid VIS shear measurements. The colorgradient bias for Euclid galaxies with different bulgetototal flux ratio at different redshifts can be well calibrated to the level of 0.1% using galaxies from the CSST deep survey.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 January 2023
 DOI:
 10.1051/00046361/202243978
 arXiv:
 arXiv:2210.16341
 Bibcode:
 2023A&A...669A.128L
 Keywords:

 dark energy;
 dark matter;
 gravitational lensing: weak;
 largescale structure of Universe;
 surveys;
 telescopes;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 18 pages, 19 figures and 2 tables. Accepted for publication in A&