The Arizona Molecular ISM Survey with the SMT: The Diverse Carbon Monoxide Line Ratios and Spectral Line Energy Distributions of Star-forming Galaxies

The carbon monoxide (CO) spectral line energy distributions (SLEDs) of galaxies contain a wealth of information about conditions in their cold interstellar gas. Here, we use galaxy-scale observations of the three lowest energy CO lines to determine SLEDs and line ratios in a sample of 47 nearby, predominantly star-forming galaxies. We find a systematic trend of higher gas excitation with increasing star formation rate (SFR) and SFR surface density (Σ_SFR), with the range of variations being even larger than predicted by simulations. Power-law fits of the CO line ratios as a function of SFR and Σ_SFR provide a good description of the trends seen in our sample and also accurately predict values for a wide range of galaxy types compiled from the literature. Based on these fits, we provide prescriptions for estimating CO(1-0) luminosities and molecular gas masses using CO(3-2) or CO(2-1) in cases where CO(1-0) is not observed directly. We compare our observed SLEDs with molecular cloud models in order to examine how the physical properties of cold gas vary across the galaxy population. We find that gas conditions in star-forming and starburst galaxies lie on a continuum with increasing gas density in more actively star-forming systems.