The PAU Survey & Euclid: Improving broad-band photometric redshifts with multi-task learning

TL;DR

This paper introduces a multi-task learning approach that leverages narrow-band photometry during training to significantly improve the accuracy and bias of broadband photometric redshifts for large galaxy surveys, aiding cosmological studies.

Contribution

The study demonstrates that multi-task learning with narrow-band data enhances broadband photo-z accuracy and reduces outliers, especially for high-redshift galaxies, with implications for future surveys like Euclid and LSST.

Findings

Photo-z precision improved by 13% for i_{AB} < 23.

Outlier rate reduced by 40% compared to baseline.

Bias in high-redshift galaxy redshifts decreased.

Abstract

Current and future imaging surveys require photometric redshifts (photo-zs) to be estimated for millions of galaxies. Improving the photo-z quality is a major challenge but is needed to advance our understanding of cosmology. In this paper we explore how the synergies between narrow-band photometric data and large imaging surveys can be exploited to improve broadband photometric redshifts. We used a multi-task learning (MTL) network to improve broadband photo-z estimates by simultaneously predicting the broadband photo-z and the narrow-band photometry from the broadband photometry. The narrow-band photometry is only required in the training field, which also enables better photo-z predictions for the galaxies without narrow-band photometry in the wide field. This technique was tested with data from the Physics of the Accelerating Universe Survey (PAUS) in the COSMOS field. We find that the method predicts photo-zs that are 13% more precise down to magnitude i_{AB} < 23; the outlier rate is also 40% lower when compared to the baseline network. Furthermore, MTL reduces the photo-z bias for high-redshift galaxies, improving the redshift distributions for tomographic bins with z>1. Applying this technique to deeper samples is crucial for future surveys such as \Euclid or LSST. For simulated data, training on a sample with i_{AB} <23, the method reduces the photo-z scatter by 16% for all galaxies with i_{AB}<25. We also studied the effects of extending the training sample with photometric galaxies using PAUS high-precision photo-zs, which reduces the photo-z scatter by 20% in the COSMOS field.