Context. Dense gas in galactic nuclei is known to feed central starbursts and AGN, but the properties of this gas are poorly known because of the high obscuration by dust.
Aims: Submm-wave spectroscopy of water and its associated ions is useful to trace the oxygen chemistry of interstellar gas, in particular to constrain its ionization rate.
Methods: We present Herschel/HIFI spectra of the H2O 1113 GHz and H2O+ 1115 GHz lines toward five nearby prototypical starburst/AGN systems, and OH+ 971 GHz spectra toward three of these. The beam size of 20” corresponds to resolutions between 0.35 and 7 kpc.
Results: The observed line profiles range from pure absorption (NGC 4945, M 82) to P Cygni indicating outflow (NGC 253, Arp 220) and inverse P Cygni indicating infall (Cen A). The similarity of the H2O, OH+, and H2O+ profiles to each other and to HI indicates that diffuse and dense gas phases are well mixed. We estimate column densities assuming negligible excitation (for absorption features) and using a non-LTE model (for emission features), adopting calculated collision data for H2O and OH+, and rough estimates for H2O+. Column densities range from ~1013 to ~1015 cm-2 for each species, and are similar between absorption and emission components, indicating that the nuclear region does not contribute much to the emission in these ground-state lines. The N(H2O)/N(H2O+) ratios of 1.4-5.6 indicate an origin of the lines in diffuse gas, and the N(OH+)/N(H2O+) ratios of 1.6-3.1 indicate a low H2 fraction (≈11%) in the gas. The low H2O abundance relative to H2 of ~10-9 may indicate enhanced photodissociation by UV fromyoung stellar populations, or freeze-out of H2O molecules onto dust grains.
Conclusions: We use our observations to estimate cosmic-ray ionization rates for our sample galaxies, adopting recent Galactic values for the average gas density and the ionization efficiency. We find ζCR~ 3 × 10-16 s-1, similar to the value for the Galactic disk, but ~10× below that of the Galactic Center and ~100× below estimates for AGN from excited-state H3O+ lines. We conclude that the ground-state lines of water and its associated ions probe primarily non-nuclear gas in the disks of these centrally active galaxies. Our data thus provide evidence for a decrease in ionization rate by a factor of ~10 from the nuclei to the disks of galaxies, as found before for the Milky Way.
