2012MNRAS.421.1298C

Surveying 18 ¹²CO-bright galaxies from the ATLAS^3D early-type galaxy sample with the Institut de Radio Astronomie Millimétrique (IRAM) 30-m telescope, we detect ¹³CO(1–0) and ¹³CO(2–1) in all 18 galaxies, HCN(1–0) in 12/18 and HCO⁺(1–0) in 10/18. We find that the line ratios ¹²CO(1–0)/¹³CO(1–0) and ¹²CO(1–0)/HCN(1–0) are clearly correlated with several galaxy properties: total stellar mass, luminosity-weighted mean stellar age, molecular-to-atomic gas ratio, dust temperature and dust morphology. We suggest that these correlations are primarily governed by the optical depth in the ¹²CO lines; interacting, accreting and/or starbursting early-type galaxies have more optically thin molecular gas while those with settled dust and gas discs host optically thick molecular gas. The ranges of the integrated line intensity ratios generally overlap with those of spirals, although we note some outliers in the ¹²CO(1–0)/¹³CO(1–0), ¹²CO(2–1)/¹³CO(2–1) and HCN/HCO⁺(1–0) ratios. In particular, three galaxies are found to have very low ¹²CO(1–0)/¹³CO(1–0) and ¹²CO(2–1)/¹³CO(2–1) ratios. Such low ratios may signal particularly stable molecular gas which creates stars less efficiently than `normal' (i.e. below Schmidt–Kennicutt prediction), consistent with the low dust temperatures seen in these galaxies.