The ionization rates of galactic nuclei and disks from Herschel/HIFI observations of water and its associated ions

摘要

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 H _(2) O 1113 GHz and H _(2) O ~(+) 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 H _(2) O, OH ~(+) , and H _(2) O ~(+) 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 H _(2) O and OH ~(+) , and rough estimates for H _(2) O ~(+) . Column densities range from ~ 10 ~(13) to ~ 10 ~(15) 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 (H _(2) O)/ N (H _(2) O ~(+) ) ratios of 1.4 ? 5.6 indicate an origin of the lines in diffuse gas, and the N (OH ~(+) )/ N (H _(2) O ~(+) ) ratios of 1.6 ? 3.1 indicate a low H _(2) fraction ( ≈ 11%) in the gas. The low H _(2) O abundance relative to H _(2) of ~ 10 ~(-9) may indicate enhanced photodissociation by UV fromyoung stellar populations, or freeze-out of H _(2) O 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 ζ _(CR) ~ 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 H _(3) O ~(+) 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.