Photo-z Performance for Precision Cosmology II : Empirical Verification

TL;DR

This study evaluates the impact of template error on photometric redshift accuracy for large surveys, finding it to be a minor factor and demonstrating the potential for precise redshift binning with limited spectroscopic data.

Contribution

It incorporates real COSMOS photometry to assess template error effects, advancing the understanding of photo-z performance in realistic survey conditions.

Findings

Template error has a small impact on photo-z accuracy.

Photometric redshifts can achieve the required precision with limited spectroscopic training.

Simulated redder photometry effectively models high-redshift galaxy observations.

Abstract

The success of future large scale weak lensing surveys will critically depend on the accurate estimation of photometric redshifts of very large samples of galaxies. This in turn depends on both the quality of the photometric data and the photo-z estimators. In a previous study, (Bordoloi et al. 2010) we focussed primarily on the impact of photometric quality on photo-z estimates and on the development of novel techniques to construct the N(z) of tomographic bins at the high level of precision required for precision cosmology, as well as the correction of issues such as imprecise corrections for Galactic reddening. We used the same set of templates to generate the simulated photometry as were then used in the photo-z code, thereby removing any effects of "template error". In this work we now include the effects of "template error" by generating simulated photometric data set from actual COSMOS photometry. We use the trick of simulating redder photometry of galaxies at higher redshifts by using a bluer set of passbands on low z galaxies with known redshifts. We find that "template error" is a rather small factor in photo-z performance, at the photometric precision and filter complement expected for all-sky surveys. With only a small sub-set of training galaxies with spectroscopic redshifts, it is in principle possible to construct tomographic redshift bins whose mean redshift is known, from photo-z alone, to the required accuracy of 0.002(1+z).