2025ApJ...979..228K

The J = 1 → 0 spectral line of carbon monoxide (CO(1-0)) is the canonical tracer of molecular gas. However, CO(2-1) is frequently used in its place, following the assumption that the higher-energy line can be used to infer the CO(1-0) luminosity and molecular gas mass. The use of CO(2-1) depends on a knowledge of the ratio between CO(2-1) and CO(1-0) luminosities, r₂₁. Here, we present galaxy-integrated r₂₁ measurements for 122 galaxies spanning stellar masses from 10⁹ to 10^11.5M_⊙ and star formation rates (SFRs) from 0.08 to 35 M_⊙ yr^–1. We find strong trends between r₂₁ and SFR, SFR surface density, star formation efficiency, and distance from the star formation main sequence (SFMS). We show that the assumption of a constant r₂₁ can introduce biases into the molecular gas trends in galaxy population studies and demonstrate how this affects the recovery of important galaxy scaling relations, including the Kennicutt-Schmidt law and the relation between SFMS offset and star formation efficiency. We provide a prescription that accounts for variations in r₂₁ as a function of SFR and can be used to convert between CO(2-1) and CO(1-0) when only one line is available. Our prescription matches variations in r₂₁ for both AMISS and literature samples and can be used to derive more accurate gas masses from CO(2-1) observations.