Letter to the Editor

GLIMPSE-D: Metallicity decline in faint galaxies: Implications for [O III]+Hβ luminosity function and re-ionisation budget

¹ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
² CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Rutgers University, Department of Physics and Astronomy, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA
⁵ Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, CRAL UMR5574, F-69230 Saint-Genis-Laval, France
⁶ Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden
⁷ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada
⁸ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada
⁹ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
¹⁰ Cosmic Frontier Center, The University of Texas at Austin, Austin, TX 78712, USA
¹¹ Institut d’Astrophysique de Paris, UMR 7095, CNRS, Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France
¹² Department of Physics & Astronomy, Tufts University, MA 02155, USA
¹³ MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
¹⁴ Department of Astronomy, The Oskar Klein Centre, Stockholm University, AlbaNova SE-10691 Stockholm, Sweden

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 January 2026
Accepted: 11 May 2026

Abstract

We report the measurement of the R3 = [O III] λ5008/Hβ ratios for 54 galaxies in the GLIMPSE-D survey. Thanks to gravitational lensing, our sample includes galaxies with −20 ≲ M_UV < −14 at z = 6 − 9. We derived oxygen abundances using calibrated relationships. We observe a significant decline in R3 values below M_UV ≳ −18, which we interpret as evidence of decreasing metallicities in fainter regimes. We explored four prescription models of the evolution of R3 with UV emission based on the new measurements and results from previous surveys. Applying these models to the GLIMPSE [O III]+Hβ luminosity functions, we measured and extrapolated the ionising photon-production rate, Ṅ_ion of galaxies down to very faint limits (SFR_Hα ≳ 5 × 10⁻³ M_⊙ yr⁻¹). If our results can be generalised, they indicate a dominant contribution of star-forming galaxies to re-ionisation and are consistent with the recent discovery of ultra-faint metal-poor galaxies. Our measurements of the relative contribution of each luminosity bin show that galaxies with L_Hα ≈ 10^{41 − 42} erg s⁻¹ dominate at 8 < z < 9, but the relative contributions become more uniform at 7 < z < 8. Extreme models either under- or over-estimate the ionising photon budget, while intermediate models align with recent observational constraints.