Accuracy of photometric redshifts for future weak lensing surveys from space

TL;DR

This study evaluates the accuracy of photometric redshifts for future space-based weak lensing surveys using realistic simulations of multi-band observations, emphasizing the importance of u-band data and good seeing conditions.

Contribution

It provides a comprehensive simulation-based assessment of photometric redshift performance for space and ground-based surveys, highlighting key observational requirements.

Findings

u-band observations are crucial for redshift discrimination

Good seeing (<1 arcsec) significantly improves photo-z accuracy

Excluding at least 15% of galaxies achieves <0.05 photo-z precision

Abstract

Photometric redshifts are a key tool to extract as much information as possible from planned cosmic shear experiments. In this work we aim to test the performances that can be achieved with observations in the near-infrared from space and in the optical from the ground. This is done by performing realistic simulations of multi-band observations of a patch of the sky, and submitting these mock images to software usually applied to real images to extract the photometry and then a redshift estimate for each galaxy. In this way we mimic the most relevant sources of uncertainty present in real data analysis, including blending and light pollution between galaxies. As an example we adopt the infrared setup of the ESA-proposed Euclid mission, while we simulate different observations in the optical, modifying filters, exposure times and seeing values. Finally, we consider directly some future ground-based experiments, such as LSST, Pan-Starrs and DES. The results highlight the importance of u-band observations, especially to discriminate between low (z < 0.5) and high (z ~ 3) redshifts, and the need for good observing sites, with seeing FWHM < 1. arcsec. The former of these indications clearly favours the LSST experiment as a counterpart for space observations, while for the other experiments we need to exclude at least 15 % of the galaxies to reach a precision in the photo-zs equal to $<\frac{\sigma_z}{1+z}>$ < 0.05.