The ATLAS3D project - XI. Dense molecular gas properties of CO-luminous early-type galaxies

Surveying 18 (CO)-C-12-bright galaxies from the ATLAS(3D) early-type galaxy sample with the Institut de Radio Astronomie Millimetrique (IRAM) 30-m telescope, we detect (CO)-C-13(1-0) and (CO)-C-13(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 (CO)-C-12(1-0)/(CO)-C-13(1-0) and (CO)-C-12(1-0)/HCN(1-0) are clearly correlated with several galaxy properties: total stellar mass, luminosity-weighted mean stellar age, molecularto-atomic gas ratio, dust temperature and dust morphology. We suggest that these correlations are primarily governed by the optical depth in the (CO)-C-12 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 (CO)-C-12(1-0)/(CO)-C-13(1-0), (CO)-C-12(2-1)/(CO)-C-13(2-1) and HCN/HCO+(1-0) ratios. In particular, three galaxies are found to have very low (CO)-C-12(1-0)/(CO)-C-13(1-0) and (CO)-C-12(2-1)/(CO)-C-13(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.