2020MNRAS.494.1095L

We present 1 and 3 mm observations of the ¹³C- and ¹⁵N-bearing isotopologues of dense gas tracers towards eight nearby infrared-bright galaxies. With the Institut de Radioastronomie Millimetrique 30-m telescope, we observed the J = 1-0 transitions of H¹³CN, HC¹⁵N, H¹³CO⁺, HN¹³C, and H¹⁵NC towards M 82, NGC 3079, IC 694, Mrk 231, and NGC 6240. The J = 3-2 transition of H¹³CN was obtained in M 82, NGC 3079, NGC 3504, NGC 4418, NGC 6240, and NGC 6946, using the 10-m Submillimeter Telescope (SMT). We report the first detections of HN¹³C J = 1-0 and H¹³CN J = 3-2 in M 82, and H¹³CN J = 3-2 in NGC 6240 and NGC 3079. We find different line profiles between the J = 1-0 and 3-2 transitions of H¹³CN in both M 82 and NGC 3079. The optical depths of HCN show significant variations among the sample, indicating that dense gas masses estimated from the line luminosities of HCN J = 1-0 and 3-2 should be treated with caution for individual galaxies. Optical depth of HCN J = 3-2 is found to be higher than that of HCN J = 1-0 in M 82, NGC 3079, NGC 4418, and NGC 6240, which indicates that ground state transitions of dense gas tracers might better trace the star-forming gas than the high-J transitions. Based on the H¹³CN/HC¹⁵N line ratios, with the double-isotopic method, low ¹⁴N/¹⁵N abundance ratios of 120 and 140 are found in NGC 3079 and Mrk 231, respectively.