TOPz: Photometric redshifts using template fitting applied to the GAMA survey

¹ Tartu Observatory, University of Tartu, Observatooriumi 1, 61602 Tõravere, Estonia
² Estonian Academy of Sciences, Kohtu 6, 10130 Tallinn, Estonia

^★ Corresponding author: elmo.tempel@ut.ee

Received: 6 January 2025
Accepted: 5 October 2025

Abstract

Context. Accurate photometric redshift (photo-z) estimation is crucial for cosmological and galaxy evolution studies, especially with the advent of large-scale photometric surveys.

Aims. We developed a photo-z estimation code called TOPz (Tartu Observatory Photo-z) and applied it to the GAMA photometric catalogue. Using nine-band photometric data from the GAMA project, we assessed the accuracy of TOPz by comparing its photo-z estimates to available spectroscopic redshifts from GAMA and DESI DR1. The latter extends to z < 2 and m_Z < 24, allowing the photo-z accuracy to be validated beyond the GAMA limits.

Methods. TOPz employs a Bayesian template-fitting approach to estimate photo-z from marginalised redshift posteriors. We generated synthetic galaxy spectra using the CIGALE software and ran template set optimisation. We improved the photometry by applying flux and flux uncertainty corrections. An analytical prior was then imposed on the resulting posteriors to refine the redshift estimates.

Results. The photo-z estimates produced by TOPz show good agreement with the spectroscopic redshifts in the low-redshift regime (z < 0.5) where the majority (95%) of the GAMA spectroscopic redshifts are. We demonstrate the redshift accuracy across various magnitude bins and tested how the flux corrections and posteriors reflect the actual uncertainty of the estimates. For the GAMA sample, the σ_NMAD = 0.012 for m_Z < 18 and increases to σNMAD = 0.021 for m_Z > 19. The outlier fraction (|∆z|/(1 + z) > 0.1) in the same magnitude bins increases from 1 to 5%. We show that the TOPz results are consistent with those obtained from other photo-z codes (EAZY and SFM) applied to the same data set. Additionally, TOPz estimates stellar masses as a by-product, comparable to those calculated by other methods. We have made the full GAMA photo-z catalogue and all the codes and scripts used for the analysis and figures publicly available.

Conclusions. TOPz is an advanced photo-z estimation code that integrates flux corrections, physical priors, and template set optimisation to provide state-of-the-art photo-z among competing template-based redshift estimators. Future work will focus on incorporating additional photometric data and applying the TOPz algorithm to the J-PAS narrow-band survey, further validating and enhancing its capabilities.