2019ApJ...882...95E

Centimeter-wave transitions are important counterparts to the rotational millimeter-wave transitions usually observed to study gas in star-forming regions. However, given their relative weakness, these transitions have historically been neglected. We present Australia Telescope Compact Array 4 cm and 15 mm band spectral line observations of nine nearby star-forming galaxies in the H75 array configuration. Thirteen different molecular lines are detected across the sample from OH, NH₃, H₂O, H₂CO, and c-C₃H₂, as well as 18 radio recombination lines (RRLs) in NGC 253. Excited OH ²Π_3/2 absorption is detected toward NGC 253 (J = 5/2), NGC 4945 (J = 9/2), and Circinus (J = 9/2); the latter two represent only the third and fourth extragalactic J = 9/2 detections. These lines in Circinus suggest rotation temperatures in excess of 2000 K, and thus it is likely that the populations of OH rotational states are not governed by a Boltzmann distribution. Circinus's OH lines are blueshifted from the systemic velocity by ∼35 km s^–1, while NGC 4945's are redshifted by ∼100 km s^–1. NGC 4945's OH absorption likely indicates infall onto the nucleus. The NH₃ (1,1) through (6,6) lines in NGC 4945 display a superposition of emission and absorption similar to that seen in other dense gas tracers. Strong (3,3) emission points toward maser activity. The relative NH₃ absorption strengths in NGC 4945 show similar anomalies as in previous studies of Arp 220 (weak (1,1) and strong (5,5) absorption). A trend toward higher local thermal equilibrium electron temperatures with increasing RRL frequency is present in NGC 253, likely indicative of stimulated emission within the nuclear region.