2019MNRAS.482.4763L

We present results of millimetre line observations towards four local gas-rich galaxies (NGC 3079, NGC 4258, NGC 6240, and VII Zw 31) with the IRAM 30m millimetre telescope. More than 33 lines in these four sources were detected, including normal dense gas tracers (HCN 1-0, HCO⁺ 1-0, and C₂H 1-0, etc.) and their isotopic species. H¹³CN (1-0) and H¹³CO⁺ (1-0) are detected for the first time in NGC 4258. Optical depths of HCN 1-0 and HCO⁺ 1-0 were estimated with detected isotopic lines in NGC 4258, which were 4.1 and 2.6, respectively. HC₃N J = 29-28, which requires high-volume density and high temperature to excite, was detected in NGC 6240. High ratios of HCO⁺/HCN in NGC 4258 and NGC 6240 imply that this ratio might not be a perfect diagnostic tool between active galactic nucleus and starburst environments, due to contamination/combination of both processes. The low HC₃N/HCN line ratios with less than 0.15 in NGC 4258, NGC 6240 and the non-detection of HC₃N line in NGC 3079 and VII Zw 31 indicate that these four galaxies are HC₃N-poor galaxies. The variation of fractional abundance of CN in different types of galaxies is large.