Optimising the shape of photometric redshift distributions with clustering cross-correlations

TL;DR

This paper introduces an optimization method combining simulated annealing and self-organising maps to improve the assignment of galaxies to redshift bins, significantly reducing outliers and enhancing the accuracy of photometric redshift distributions.

Contribution

The novel approach integrates simulated annealing with SOMs to optimize galaxy redshift binning based on clustering signals, improving photometric redshift distribution accuracy.

Findings

Significant reduction in outlier fraction, especially at high redshift.

Method improves the compactness of redshift distributions.

Demonstrated effectiveness on synthetic LSST cosmoDC2 data.

Abstract

We present an optimisation method for the assignment of photometric galaxies into a chosen set of redshift bins. This is achieved by combining simulated annealing, an optimisation algorithm inspired by solid-state physics, with an unsupervised machine learning method, a self-organising map (SOM) of the observed colours of galaxies. Starting with a sample of galaxies that is divided into redshift bins based on a photometric redshift point estimate, the simulated annealing algorithm repeatedly reassigns SOM-selected subsamples of galaxies, which are close in colour, to alternative redshift bins. We optimise the clustering cross-correlation signal between photometric galaxies and a reference sample of galaxies with well-calibrated redshifts. Depending on the effect on the clustering signal, the reassignment is either accepted or rejected. By dynamically increasing the resolution of the SOM, the algorithm eventually converges to a solution that minimises the number of mismatched galaxies in each tomographic redshift bin and thus improves the compactness of their corresponding redshift distribution. This method is demonstrated on the synthetic LSST cosmoDC2 catalogue. We find a significant decrease in the fraction of catastrophic outliers in the redshift distribution in all tomographic bins, most notably in the highest redshift bin with a decrease in the outlier fraction from 57 per cent to 16 per cent.