Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H2O, H2^{18}O, OH,
  ^{18}OH, O I, HCN and NH3
  Herschel/PACS spectroscopy of the luminous infrared galaxies NGC4418 and
Arp220 reveals high excitation in H2O, OH, HCN, and NH3. In NGC4418, absorption
lines were detected with E_low>800 K (H2O), 600 K (OH), 1075 K (HCN), and 600 K
(NH3), while in Arp220 the excitation is somewhat lower. While outflow
signatures in moderate excitation lines are seen in Arp220 as reported in
previous studies, in NGC4418 the lines tracing its outer regions are redshifted
relative to the nucleus, suggesting an inflow with Mdot<~12 Msun yr^{-1}. Both
galaxies have warm (Tdust>~100 K) nuclear continuum components, together with a
more extended component that is much more prominent and massive in Arp220. A
chemical dichotomy is found in both sources: on the one hand, the nuclear
regions have high H2O abundances, ~10^{-5}, and high HCN/H2O and HCN/NH3 column
density ratios of 0.1-0.4 and 2-5, respectively, indicating a chemistry typical
of evolved hot cores where grain mantle evaporation has occurred. On the other
hand, the high OH abundance, with OH/H2O ratios of ~0.5, indicates the effects
of X-rays and/or cosmic rays. The nuclear media have surface brightnesses
>~10^{13} Lsun/kpc^2 and are estimated to be thick (N_H>~10^{25} cm^{-2}).
While NGC4418 shows weak absorption in H2^{18}O and ^{18}OH, with a
^{16}O-to-^{18}O ratio of >~250-500, the strong absorption of the rare
isotopologues in Arp220 indicates ^{16}O-to-^{18}O of 70-130. Further away from
the nuclear regions, the H2O abundance decreases to <~10^{-7} and the OH/H2O
ratio is 2.5-10. Despite the different scales of NGC4418, Arp220, and Mrk231,
preliminary evidence is found for an evolutionary sequence from infall,
hot-core like chemistry, and solar oxygen isotope ratio to high velocity
outflow, disruption of the hot core chemistry and cumulative high mass stellar
processing of 18O.
