ALMA ACE Survey Shows Stellar Mass Dominates Gas-Phase Metallicity at z~2 with 0.10 dex Scatter

The ALMA Chemical Evolution survey targeted CO(3-2) emission in 26 star-forming galaxies at z~2, securing detections in 17 sources including some of the lowest-metallicity CO detections at this epoch. Gas masses were derived from CO luminosities while metallicities came from optical strong-line calibrations, allowing direct construction of the gas-phase fundamental metallicity relation. Stellar mass alone accounts for most of the observed scatter at 0.10 dex, while adding molecular gas mass or SFR reduces scatter only marginally to 0.11-0.13 dex, consistent with the sample probing primarily the CO-bright molecular phase.

These results align with gas-regulator models that invoke efficient molecular outflows to regulate both star formation and chemical enrichment. An average mass-loading factor of eta~4 is inferred, sufficient to eject metal-enriched gas and maintain the observed mass-metallicity slope. The findings extend the local fundamental metallicity relation to higher redshift while highlighting that the molecular-gas term remains secondary when only the dense, CO-traced reservoir is measured.

Future ALMA and JWST programs targeting fainter CO isotopologues and [CII] will test whether the full molecular reservoir strengthens the gas term. Larger samples spanning a wider metallicity baseline are required to determine whether the current ranking of drivers persists below 12+log(O/H)=8.2.