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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.

The interstellar medium of Centaurus A (NGC 5128) has been studied extensively in recent years, using mostly molecular lines tracing low to medium density gas (500 to several 10(3) cm(-3)). The amount and distribution of the dense molecular gas was largely unknown. Here we present new millimeter data of the HCN(1-0), CS(2-1), and CS(3-2) rotational transitions towards the nearby radio galaxy Centaurus A observed with the SEST on La Silla, Chile. We obtained spectra of the HCN(1-0) emission which traces dense 10(4) cm-3 molecular gas at the center and along the prominent dust lane at offset positions +/-60 " and +/-100 ". We also obtained a few spectra of CS(2-1) and (3-2) tracing densities of similar to 10(5) cm(-3). The emission in these lines is weak and reaches a few mK at the available angular resolutions of 53 " - 36 ". At the central position, the integrated intensity ratio I(HCN)/I(CO) peaks at 0.064, and decreases to similar to 0.02 to 0.04 in the dust lane.Using the new high density tracer data, we estimate the amount, distribution and physical conditions of the dense molecular gas in the dust lane of Centaurus A. We find that Cen A and the Milky Way are comparable in their HCN(1-0) line luminosity. However, towards the nucleus the fraction of dense molecular gas measured via the line luminosity ratio L(HCN)/L(CO) as well as the star formation efficiency L-FIR/L-CO is comparable to ultra-luminous infrared galaxies (ULIRGs). Within the off-nuclear dust lane and for Cen A as a whole these quantities are between those of ULIRGs and normal and infrared luminous galaxies. This suggests that most of the FIR luminosity of Centaurus A originates in regions of very dense molecular gas and high star formation efficiency.

Surveying eighteen 12CO-bright galaxies from the ATLAS3D early-type galaxy sample with the Institut de Radio Astronomie Millim\'etrique (IRAM) 30m telescope, we detect 13CO(1-0) and 13CO(2-1) in all eighteen 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., Comment: MNRAS accepted

The interstellar medium of Centaurus A (NGC 5128) has been studied extensively in recent years, using mostly molecular lines tracing low to medium density gas (500 to several $10^3$ cm$^{-3}$). The amount and distribution of the dense molecular gas was largely unknown. Here we present new millimeter data of the HCN(1-0), CS(2-1), and CS(3-2) rotational transitions towards the nearby radio galaxy Centaurus A observed with the SEST on La Silla, Chile. We obtained spectra of the HCN(1-0) emission which traces dense 10$^4$ cm$^{-3}$ molecular gas at the center and along the prominent dust lane at offset positions $\pm60^{\prime \prime}$ and $\pm100^{\prime \prime}$. We also obtained a few spectra of CS(2-1) and (3-2) tracing densities of $\sim10^5$ cm$^{-3}$. The emission in these lines is weak and reaches a few mK at the available angular resolutions of 54$^{\prime \prime}$ - 36$^{\prime \prime}$. At the central position, the integrated intensity ratio I(HCN)/I(CO) peaks at 0.064, and decreases to $\sim$0.02 to 0.04 in the dust lane., Comment: 7 pages, 5 figures, accepted by A.&A. 04. May 2000