2020MNRAS.497.2771M

We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations towards 27 low-redshift (0.02 < z < 0.2) star-forming galaxies taken from the Valparaiso ALMA/APEX Line Emission Survey. We perform stacking analyses of the ¹²CO(1-0), ¹³CO(1-0), and C¹⁸O(1-0) emission lines to explore the L' [¹²CO(1-0)]/L' [¹³CO(1-0)] [hereafter L' (¹²CO)/L' (¹³CO)] and L' [¹³CO(1-0)]/L' [C¹⁸O(1-0)] [hereafter L' (¹³CO)/L' (C¹⁸O)] line luminosity ratio dependence as a function of different global galaxy parameters related to the star formation activity. The sample has far-IR luminosities of 10^10.1–11.9 L_☉ and stellar masses of 10^9.8–10.9 M_☉ corresponding to typical star-forming and starburst galaxies at these redshifts. On average, we find an L' (¹²CO)/L' (¹³CO) line luminosity ratio value of 16.1 ± 2.5. Galaxies with pieces of evidence of possible merging activity tend to show higher L' (¹²CO)/L' (¹³CO) ratios by a factor of 2, while variations of this order are also found in galaxy samples with higher star formation rates (SFRs) or star formation efficiencies (SFEs). We also find an average L' (¹³CO)/L' (C¹⁸O) line luminosity ratio of 2.5 ± 0.6, which is in good agreement with those previously reported for starburst galaxies. We find that galaxy samples with high L_IR, SFR, and SFE show low L' (¹³CO)/L' (C¹⁸O) line luminosity ratios with high L' (¹²CO)/L' (¹³CO) line luminosity ratios, suggesting that these trends are produced by selective enrichment of massive stars in young starbursts.