Photo-z Performance for Precision Cosmology

TL;DR

This paper evaluates the feasibility of achieving the high-precision photometric redshifts needed for future cosmological surveys like Euclid, demonstrating that with combined near-infrared and ground-based data, the required accuracy is attainable despite challenges.

Contribution

It provides a comprehensive analysis of photo-z performance requirements for Euclid, proposing methods to meet precision and outlier rejection criteria using combined photometry and statistical techniques.

Findings

Photo-z precision of sigma(z) < 0.05(1+z) achievable with Euclid and ground-based data.

Outlier fraction can be reduced to < 0.25% with simple rejection schemes.

Mean redshift <z> can be accurately estimated using photo-z likelihood functions.

Abstract

Current and future weak lensing surveys will rely on photometrically estimated redshifts of very large numbers of galaxies. In this paper, we address several different aspects of the demanding photo-z performance that will be required for future experiments, such as the proposed ESA Euclid mission. It is first shown that the proposed all-sky near-infrared photometry from Euclid, in combination with anticipated ground-based photometry (e.g. PanStarrs-2 or DES) should yield the required precision in individual photo-z of sigma(z) < 0.05(1+z) at I_AB < 24.5. Simple a priori rejection schemes based on the photometry alone can be tuned to recognise objects with wildly discrepant photo-z and to reduce the outlier fraction to < 0.25% with only modest loss of otherwise usable objects. Turning to the more challenging problem of determining the mean redshift <z> of a set of galaxies to a precision of 0.002(1+z) we argue that, for many different reasons, this is best accomplished by relying on the photo-z themselves rather than on the direct measurement of <z> from spectroscopic redshifts of a representative subset of the galaxies. A simple adaptive scheme based on the statistical properties of the photo-z likelihood functions is shown to meet this stringent systematic requirement. We also examine the effect of an imprecise correction for Galactic extinction and the effects of contamination by fainter over-lapping objects in photo-z determination. The overall conclusion of this work is that the acquisition of photometrically estimated redshifts with the precision required for Euclid, or other similar experiments, will be challenging but possible. (abridged)