The Arizona Molecular ISM Survey with the SMT: Variations in the CO(2-1)/CO(1-0) Line Ratio Across the Galaxy Population

TL;DR

This study measures the CO(2-1)/CO(1-0) line ratio across 122 galaxies, revealing its dependence on galaxy properties and providing a correction method to improve molecular gas mass estimates from CO observations.

Contribution

It presents the first extensive galaxy-wide measurements of r$_{21}$, demonstrating its variability and impact on molecular gas and star formation studies, along with a new correction prescription.

Findings

r$_{21}$ varies with SFR, surface density, and efficiency

Constant r$_{21}$ assumptions bias gas mass and star formation relations

A new SFR-dependent correction improves molecular gas mass estimates

Abstract

The J=1$\rightarrow$0 spectral line of carbon monoxide (CO(1-0)) is the canonical tracer of molecular gas. However, CO(2-1) is frequently used in its place, following the assumption that the higher energy line can be used to infer the CO(1-0) luminosity and molecular gas mass. The use of CO(2-1) depends on a knowledge of the ratio between CO(2-1) and CO(1-0) luminosities, r$_{21}$. Here we present galaxy-integrated r$_{21}$ measurements for 122 galaxies spanning stellar masses from 10$^9$ to 10$^{11.5}$ M$_\odot$ and star formation rates (SFRs) from 0.08 to 35 M$_\odot$/yr. We find strong trends between r$_{21}$ 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 r$_{21}$ 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 which accounts for variations in r$_{21}$ 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 r$_{21}$ for both AMISS and literature samples and can be used to derive more accurate gas masses from CO(2-1) observations.