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
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.
