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Herschel/PACS spectroscopy of the luminous infrared galaxies NGC4418 andArp220 reveals high excitation in H2O, OH, HCN, and NH3. In NGC4418, absorptionlines 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 outflowsignatures in moderate excitation lines are seen in Arp220 as reported inprevious studies, in NGC4418 the lines tracing its outer regions are redshiftedrelative to the nucleus, suggesting an inflow with Mdot<~12 Msun yr^{-1}. Bothgalaxies have warm (Tdust>~100 K) nuclear continuum components, together with amore extended component that is much more prominent and massive in Arp220. Achemical dichotomy is found in both sources: on the one hand, the nuclearregions have high H2O abundances, ~10^{-5}, and high HCN/H2O and HCN/NH3 columndensity ratios of 0.1-0.4 and 2-5, respectively, indicating a chemistry typicalof evolved hot cores where grain mantle evaporation has occurred. On the otherhand, the high OH abundance, with OH/H2O ratios of ~0.5, indicates the effectsof 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 rareisotopologues in Arp220 indicates ^{16}O-to-^{18}O of 70-130. Further away fromthe nuclear regions, the H2O abundance decreases to <~10^{-7} and the OH/H2Oratio 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 velocityoutflow, disruption of the hot core chemistry and cumulative high mass stellarprocessing of 18O.

Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H2O, H2^{18}O, OH, ^{18}OH, O I, HCN and NH3