1994A&A...284...17H

¹²CO, c¹⁸O, CN, HCN, N₂H⁺, and HNCO transitions, tentative detections of C₃H₂, HC₃N, and C¹⁷O transitions, and upper limits for transitions of other molecular species are reported from the nuclear region of the starburst galaxy NGC 4945. The bulk of the CO emission arises from gas with densities >1000 cm^–3. A first map in a molecular high density tracer (HCN) indicates that low and high density molecular gas coexists in the central few 100 pc; area filling factors derived from CO and HCN are ∼0.3 and ∼0.01, respectively. The presence of two main molecular components is confirmed: One is associated with gas presumably forming an edge-on nuclear ring. The other component arises from a more quiescent environment. CN, HNCO, and H₂O data indicate different physical conditions in the two outer lobes of the rotating ring. The redshifted lobe at ∼700 kms/s appears to be higher excited, giving rise to enhanced CN N=2-1 and H₂O emission. CO and HCN data also allow estimates of isotope ratios. The ¹²C/¹³C ratio is likely ∼50, suggesting recent infall of gas toward the nuclear region. The ¹⁶O/¹⁸O and ¹⁸O/¹⁷O ratios are ∼150 and ≥8. This is explained by a high ¹⁸O abundance due to recent nucleosynthesis in massive stars; the starburst must have reached an advanced stage of evolution.