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A&A 477, L5-L8 (2008)
DOI: 10.1051/0004-6361:20078824
Letter
Detection of extragalactic H
O
F. F. S. van der Tak1, 2, S. Aalto3, and R. Meijerink4 1 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
e-mail: vdtak@sron.nl
2 Kapteyn Astronomical Institute, University of Groningen, The Netherlands
3 Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
4 Astronomy Department, University of California, 601 Campbell Hall, Berkeley, CA 94720, USA
(Received 10 October 2007 / Accepted 12 November 2007 )
Abstract
Context.The H3O+ molecule probes the oxygen chemistry and the
ionization rate of dense circumnuclear gas in galaxies.
Aims.Recent H3O+ observations show variations in the
cosmic-ray ionization rate by factors of >10 within our Galaxy.
Methods.Using the JCMT, we have observed the 364 GHz line of p-H3O+ in the centers of
M 82 and Arp 220.
Results.In Arp 220, the line profile suggests that the emission originates in the Western
nucleus. In M 82, both the eastern molecular peak and the circumnuclear region
contribute to the emission. The derived column densities,
abundances, and H3O+/H2O ratios indicate ionization rates similar to
or even exceeding that in the Galactic Center.
Conclusions.Model calculations of the chemistry of irradiated molecular gas indicate a
likely origin of this high ionization rate in the extended, evolved starburst
of M 82. In contrast, irradiation by X-rays from the AGN disk is the most
likely model for Arp 220.
Key words: galaxies: starburst -- galaxies: active -- radio lines: galaxies -- ISM: molecules
© ESO 2007