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

Abstract

Centimeter-wave transitions are important counterparts to the rotational mm-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 4cm- and 15mm-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, NH$_3$, H$_2$O, H$_2$CO, and c-C$_3$H$_2$, as well as 18 radio recombination lines (RRLs) in NGC 253. Excited OH $^2\Pi_{3/2}$ absorption is detected towards 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 NH$_3$ (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 towards maser activity. The relative NH$_3$ 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 towards higher LTE electron temperatures with increasing RRL frequency is present in NGC 253, likely indicative of stimulated emission within the nuclear region.