A Spectral Analysis of the Centimeter Regime of Nearby Galaxies: RRLs, Excited OH, and NH3

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 (2)Pi(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 similar to 35 km s(-1), while NGC 4945's are redshifted by similar to 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.