Improving Optical Photo-z Constraints for Dusty Star-forming Galaxies Using Submillimeter-based Priors

Abstract

Photometric redshifts (photo-z's) provide an efficient alternative to spectroscopic redshifts, enabling redshift estimation for large galaxy samples. However, traditional photo-z methods primarily rely on optical and near-infrared (OIR) photometry, which can struggle with dusty star-forming galaxies that are often faint in the OIR but bright at far-infrared (FIR) and millimeter wavelengths. We present a method for incorporating FIR-to-millimeter photometry as a prior within standard OIR-based photo-z frameworks, explicitly folding in the observed empirical relationship between total infrared luminosity and dust temperature. This approach is particularly suitable for wide-area surveys, such as those anticipated with the Euclid satellite or Rubin Observatory, where OIR photo-z's can be complemented with longer-wavelength data to help with the dustiest and highest star-forming galaxies. Applying this method to the Herschel Astrophysical Terahertz Large Area Survey (or H-ATLAS) catalog, which combines FIR photometry from Herschel-SPIRE with OIR observations, we achieve a threefold reduction in catastrophic outliers compared to traditional OIR-based photo-z techniques, demonstrating its utility for improving redshift estimates in FIR-bright galaxies.