ALMA Chemical Evolution (ACE) survey: dust-to-gas ratios in sub-solar metallicity galaxies at cosmic noon

Dust is a fundamental component of the interstellar medium and provides a key tracer of the baryon cycle that regulates galaxy evolution. The dust-to-gas ratio links metals in the gas phase to those locked into dust grains, making it a sensitive diagnostic of dust production, grain growth, and destruction. We present measurements of the dust-to-molecular-gas ratio (DGR mol), for typical star-forming galaxies (M about 10) at sub-solar metallicites, at z 2-2.5 from the ALMA Chemical Evolution (ACE) survey. By combining ALMA CO and dust-continuum observations with robust gas-phase metallicity measurements, ACE extends direct dust and molecular-gas measurements to lower stellar masses and lower metallicities than previously available at this epoch, reaching down to 0.4,Z. This enables the first constraints on the DGR mol --metallicity relation for typical unlensed galaxies at cosmic noon. We find that DGR mol increases with metallicity, with a log-space slope of 1.2 plus or minus 0.7, indicating that metal-poor galaxies have systematically lower DGR mol than their more metal-rich counterparts. For the detected ACE galaxies, we measure a mean value of 10(M dust/M mol)=-2.37 plus or minus0.05 for a mean metallicity of 12+ (O/H) = 8.45 plus or minus 0.02. We find agreement with DGR mol in the local Universe at fixed metallicities, indicating that the same dust-growth physics, likely grain growth in the ISM, dominates at metallicities of 8.3 at most 12+ (O/H) at most 8.7 at cosmic noon. These measurements provide novel empirical constraints for models of dust enrichment and galaxy evolution during the peak epoch of cosmic star formation. Additionally, ACE provides a sub-solar metallicity reference for the calibration of dust continuum as tracer of molecular gas, essential for studying metal-poor, high-redshift systems.