Weak evolution of the mass-metallicity relation at cosmic dawn in the FirstLight simulations

Little is known about the mass-metallicity relation (MZR) in galaxies at cosmic dawn. Studying the first appearance of the MZR is one of the keys to understand the formation and evolution of the first galaxies. In order to lay the groundwork for upcoming observational campaigns, we analyse 290 galaxies in haloes spanning M_h = 10⁹-10¹¹ M_⊙ selected from the FirstLight cosmological zoom simulations to predict the MZR at z = 5-8. Over this interval, the metallicity of FirstLight galaxies with stellar mass M_* = 10⁸ M_⊙ declines by ≤0.2 dex. This contrasts with the observed tendency for metallicities to increase at lower redshifts, and reflects weakly evolving or even increasing gas fractions. We assess the use of the R3 strong-line diagnostic as a metallicity indicator, finding that it is informative for 12 + log (O/H) < 8 but saturates to R3 ≈ 3 at higher metallicities owing to a cancellation between enrichment and spectral softening. None the less, campaigns with JWST should be able to detect a clear trend between R3 and stellar mass for M_* > 10^7.5 M_⊙. We caution that, at fixed metallicity, galaxies with higher specific star formation show higher R3 owing to their more intense radiation fields, indicating a potential for selection biases.