2011MNRAS.418.1753J

We present 3-mm observations obtained with the Institut de Radioastronomie Millimétrique (IRAM) 30-m telescope for 10 nearby gas-rich active galaxies spanning three orders of magnitude in infrared luminosity. Emission lines of HCO⁺(1–0), HCN(1–0) and C₂H(1–0) were simultaneously detected in most galaxies of our sample. We also tentatively detected the rare isotopic species H¹³CN, HC¹⁵N and H¹³CO⁺ in up to four galaxies (IC 694, NGC 3690, NGC 4258 and NGC 6240). Our estimation shows that the optical depth of HCN is low to moderate in these galaxies (τ ∼ 1–5). After comparing the intensity ratios of different molecular emission lines (HCN/HCO⁺, C₂H/HCN and C₂H/HCO⁺) with the infrared luminosity (L_IR), we find that in the infrared luminous sample of five composite (AGN+starburst) galaxies these ratios vary with L_IR: the HCN/HCO⁺ ratio increases with L_IR, which is consistent with previous studies, while there is a slight trend in the composite galaxies that both C₂H/HCN and C₂H/HCO⁺ ratios decrease with increasing L_IR. Although our sample is limited, this trend may possibly imply a relation between the overall gas density and the infrared luminosity of galaxies.

Based on observations carried out with the IRAM 30-m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).