Accurate photometric redshift probability density estimation - method comparison and application

TL;DR

This paper presents an ordinal classification method for photometric redshift estimation that enhances the accuracy of individual galaxy redshift PDFs and reduces systematic biases in cosmological analyses, while requiring less storage than existing methods.

Contribution

The authors introduce an ordinal classification algorithm that improves photometric redshift PDFs and proposes a new point estimate method, offering comparable accuracy with significantly reduced storage needs.

Findings

50% improvement in log-likelihood for high redshift objects

Reduction of systematic biases by up to a factor of four

Enhanced accuracy in cosmological measurements such as lensing and correlation functions

Abstract

We introduce an ordinal classification algorithm for photometric redshift estimation, which significantly improves the reconstruction of photometric redshift probability density functions (PDFs) for individual galaxies and galaxy samples. As a use case we apply our method to CFHTLS galaxies. The ordinal classification algorithm treats distinct redshift bins as ordered values, which improves the quality of photometric redshift PDFs, compared with non-ordinal classification architectures. We also propose a new single value point estimate of the galaxy redshift, that can be used to estimate the full redshift PDF of a galaxy sample. This method is competitive in terms of accuracy with contemporary algorithms, which stack the full redshift PDFs of all galaxies in the sample, but requires orders of magnitudes less storage space. The methods described in this paper greatly improve the log-likelihood of individual object redshift PDFs, when compared with a popular Neural Network code (ANNz). In our use case, this improvement reaches 50\% for high redshift objects ($z \geq 0.75$). We show that using these more accurate photometric redshift PDFs will lead to a reduction in the systematic biases by up to a factor of four, when compared with less accurate PDFs obtained from commonly used methods. The cosmological analyses we examine and find improvement upon are the following: gravitational lensing cluster mass estimates, modelling of angular correlation functions, and modelling of cosmic shear correlation functions.