4-8 October 2015
Hans Harnack Haus
Europe/Berlin timezone
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Contribution Contributed Talk (WITHDRAWN)


Water and H2O+ in dense galactic nuclei


  • Prof. Floris VAN DER TAK

Primary authors


Dense gas in galactic nuclei is known to feed central starbursts and AGN, but the properties of this gas are poorly known due to the high obscuration by dust. Water and H2O+ are useful to trace the oxygen chemistry of interstellar gas, and its ionization rate. We present Herschel/HIFI spectra of the H2O 1113 GHz and H2O+ 1115 GHz lines toward 5 nearby prototypical starburst/AGN systems. The beam size of 20'' corresponds to resolutions between 0.35 and 7 kpc. The observed line profiles range from pure absorption (NGC 4945, M82, Arp 220) to P Cygni indicating outflow (NGC 253) and inverse P Cygni indicating infall (Cen A). The profiles of H2O and H2O+ are remarkably similar, indicating that the lines trace the same gas. 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 rough estimates for H2O+. Columns range from ~1e13 to ~1e15 cm$^{-2}$ for both species, and are similar between absorption and emission components. The H2O/H2O+ ratios are 1.4-5.6, indicating an origin of the lines in diffuse gas. However, the H2O abundance is only ~1e-9, perhaps indicating enhanced photodissociation by UV from the nuclei or depletion of H2O onto dust grains. We combine our N(H2O+) values with literature data to estimate the cosmic-ray ionization rates for our sample, adopting recent Galactic values for the average cloud density, the atomic hydrogen fraction, and the ionization efficiency. We find zeta_CR ~1e-16 s$^{-1}$, similar to the value for the Galactic disk, but somewhat below that of the Galactic center and well below that of AGN estimates from excited-state H3O+ lines. Since low filling factors appear unlikely, we conclude that the ground-state lines of H2O and H2O+ probe primarily non-nuclear gas in the disks of these centrally active galaxies.