Exploring the molecular chemistry and excitation in obscured luminous
  infrared galaxies: An ALMA mm-wave spectral scan of NGC 4418
  We obtained an ALMA Cycle 0 spectral scan of the dusty LIRG NGC 4418,
spanning a total of 70.7 GHz in bands 3, 6, and 7. We use a combined local
thermal equilibrium (LTE) and non-LTE (NLTE) fit of the spectrum in order to
identify the molecular species and derive column densities and excitation
temperatures. We derive molecular abundances and compare them with other
Galactic and extragalactic sources by means of a principal component analysis.
We detect 317 emission lines from a total of 45 molecular species, including 15
isotopic substitutions and six vibrationally excited variants. Our LTE/NLTE fit
find kinetic temperatures from 20 to 350 K, and densities between 10$^5$ and
10$^7$ cm$^{-3}$. The spectrum is dominated by vibrationally excited HC$_3$N,
HCN, and HNC, with vibrational temperatures from 300 to 450 K. We find high
abundances of HC$_3$N, SiO, H$_2$S, and c-HCCCH and a low CH$_3$OH abundance. A
principal component analysis shows that NGC 4418 and Arp 220 share very similar
molecular abundances and excitation, which clearly set them apart from other
Galactic and extragalactic environments. The similar molecular abundances
observed towards NCG 4418 and Arp 220 are consistent with a hot gas-phase
chemistry, with the relative abundances of SiO and CH$_3$OH being regulated by
shocks and X-ray driven dissociation. The bright emission from vibrationally
excited species confirms the presence of a compact IR source, with an effective
diameter $<$5 pc and brightness temperatures $>$350 K. The molecular abundances
and the vibrationally excited spectrum are consistent with a young
AGN/starburst system. We suggest that NGC 4418 may be a template for a new kind
of chemistry and excitation, typical of compact obscured nuclei (CON). Because
of the narrow line widths and bright molecular emission, NGC 4418 is the ideal
target for further studies of the chemistry in CONs.
