The ionization rates of galactic nuclei and disks from Herschel/HIFI observations of water and its associated ions⋆
1 SRON Netherlands Institute for Space
AD Groningen, The Netherlands
2 Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands
3 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Accepted: 15 June 2016
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.
Key words: astrochemistry / ISM: molecules / galaxies: ISM / galaxies: active / galaxies: starburst
© ESO, 2016