The molecular ISM in luminous infrared galaxies: a 3mm line survey of
  Arp 157
  In this work we present a search for molecular line emission in the
interacting luminous infrared galaxy Arp 157 (NGC 520). We report the detection
of 19 different lines, from 12 different molecular species in our 3mm line
survey and supporting 2mm and 1mm observations. We investigated the profiles of
the detected lines, and find all species show asymmetric emission lines, with
profiles that differ systematically as a function of critical density. We
derive the physical conditions within the gas reservoir of this system using
isotope ratios, an excitation analysis, and rotation diagrams. We suggest that
in this source we are observing a warm (Tex > 23K) edge-on, asymmetric
starburst ring, plus a less dense central molecular component, which is more
optically thin. We compare our molecular abundances with several well studied
galaxies, and with Galactic regions, in order to determine the primary driver
of the chemistry in Arp 157. We highlight some significant differences between
the abundances of molecules in this source and in the more extreme
ultra-luminous infrared galaxy Arp 220. This suggests that different physical
mechanisms may become important in driving gas chemistry at different points in
the evolution of a merger induced starburst. Of the nearby galaxies we
considered, NGC 253 (whose nuclear chemistry is dominated by large-scale
shocks) and NGC 1068 (whose nuclear chemistry is X-ray dominated) have the most
similar molecular abundances to Arp 157. The Galactic region Sgr B2(OH) (where
shock desorption of ices is thought to be important) may also have similar
abundances to Arp 157. We hence postulate that shocks and/or the generation of
hard radiation fields by the ongoing merger (or a starburst wind), are driving
the molecular chemistry in Arp 157.
