Abstract
We show that the12CO/13CO intensity ratio in nearby galaxies varies systematically as a function of the star formation rate surface density and gas surface density. The same effect is observed in different transitions, and in the 12CO/C18O ratio, while the 13CO/C18O ratio appears to remain constant as a function of the star formation rate surface density. We discuss the cause of these variations, considering both changes in the physical state of the gas, and chemical changes that lead to abundance variations. We used the observed correlations with C18O to suggest that abundance variations are unlikely to be causing the systematic trend observed with the star formation rate surface density, and thus that the mean gas temperature and/or velocity dispersion are systematically higher in higher star-formation rate surface density regions. We present the best fitting relations between the star formation rate surface density and the 12CO/13CO and 12CO/C18O ratios, and discuss how this effect can help us predict CO isotope emission from galaxies across the known universe.
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