Impact of sub-solar metallicities on photometric redshifts

TL;DR

This study investigates how using solar metallicity templates biases photometric redshift estimates for low-metallicity galaxies in deep surveys, revealing a systematic underestimation of redshifts up to 0.2 for distant faint galaxies.

Contribution

It introduces chemically consistent galaxy templates and demonstrates the impact of metallicity mismatch on photometric redshift accuracy.

Findings

Significant bias in photometric redshifts due to metallicity mismatch.

Underestimation of redshifts by Delta z ~ 0.1 to 0.2 for low-metallicity galaxies.

Bias persists up to redshift z ~ 1.2.

Abstract

With the advent of deep photometric surveys the use of photometric redshifts, obtained with a variety of techniques, has become more and more widespread. Giving access to galaxies with a wide range of luminosities out to high redshifts, these surveys include many faint galaxies with significantly sub-solar metallicities. We use our chemically consistent galaxy evolutionary synthesis code GALEV to produce a large grid of template Spectral Energy Distributions (SEDs) for galaxies of spectral types E and Sa through Sd - one accounting in a chemically consistent way for the increasing initial metallicities of successive stellar generations, the other one for exclusively solar metallicities - for comparison. We use our new photometric redshift code GAZELLE based on the comparison of observed and model SEDs. Comparing the photometric redshifts obtained using solar metallicity templates when working on a catalogue of artificially created chemically consistent SEDs, typical for low-metallicity local late-type galaxies and for intrinsically low-luminosity, and hence low-metallicity, galaxies in the high-redshift universe, we find a significant bias resulting from this metallicity mismatch. This bias consists in a systematic underestimate of the photometric redshift by typically Delta z ~ 0.1 ... 0.2 until z ~ 1.2, depending on galaxy type, of distant, faint and low-metallicity galaxies if analysed with solar-metallicity templates.