A Spectral Analysis of the Centimeter Regime of Nearby Galaxies: RRLs,
  Excited OH, and NH$_3$
  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.
