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

Abstract: 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.