The ALPINE–CRISTAL–JWST Survey: The Fast Metal Enrichment of Massive Galaxies at z ∼ 5

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Abstract We present the stellar mass–metallicity relation (MZR) and mass–metallicity–star formation relation (“fundamental metallicity relation”; FMR) of 18 massive ( log ( M ⋆ / M ⊙ ) = 9.5 − 11 ) main-sequence galaxies at z ∼ 5 from the ALPINE–CRISTAL–JWST sample. This sample complements recent studies by JWST at up to 2 orders of magnitude lower stellar masses. The metallicities are derived using strong optical lines and verified by temperature-based oxygen abundance measurements for five galaxies for which faint auroral lines are detected. We find that the metal abundance evolves, on average, from 40% to 60% solar between z ∼ 5 and cosmic noon ( z ∼ 2) at the massive end of the MZR, suggesting already significant metal enrichment at early times. The FMR at z = 5 exhibits a 5× larger scatter (preferentially to lower metallicities) compared to the local FMR relation. This scatter can be explained by a bursty star formation and the direct buildup of metals in early galaxies, as well as differences in age and outflow efficiencies. Capitalizing on all available samples, we find that the observed MZR and FMR over 3 orders of stellar mass is generally in good agreement with results from cosmological simulations, although some underestimate the metal enrichment at low stellar masses. This may be due to too efficient metal-rich outflows. We show that the ALPINE–CRISTAL–JWST galaxies likely joined the current FMR at z ∼ 10 and will evolve into massive ( log ( M ⋆ / M ⊙ ) ∼ 11.4 ) galaxies with supersolar metallicities by z = 0.

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