GLIMPSE-D: Metallicity Decline in Faint Galaxies: Implications for [O III]+Hb Luminosity Function and Reionisation Budget

TL;DR

This study measures the decline in metallicity of faint high-redshift galaxies using [O III]+Hb ratios, revealing their significant role in cosmic reionisation and refining models of ionising photon production.

Contribution

It provides new measurements of R3 ratios in faint galaxies at z=6-9 and explores models of metallicity evolution impacting reionisation estimates.

Findings

Metallicity declines in galaxies with Muv > -18 at high redshift.

Star-forming galaxies with L(Ha)~1e41 to 1e42 erg/s dominate reionisation at 8<z<9.

Intermediate models of R3 evolution align with observational constraints.

Abstract

We report the measurement of the R3=[O III]5008/Hb ratios for 54 galaxies in the GLIMPSE-D survey. Thanks to gravitational lensing, our sample includes galaxies with -20 < Muv < -14 at z=6-9. We derive oxygen abundances using calibrated relationships. We observe a significant decline in R3 values below Muv > -18, which we interpret as evidence of decreasing metallicities in fainter regimes. We explore 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]+Hb luminosity functions, we measure and extrapolate the ionising photon production rate $\dot{N}_{ion}$ of galaxies down to very faint limits SFR(Ha) > 5e-3 Msun/yr. Our results support the dominant contribution of star-forming galaxies to reionisation, 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(Ha)~1e41 to 1e42 erg/s 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.