Letter to the Editor
Detection of extragalactic H3O
SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands e-mail: email@example.com
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
Accepted: 12 November 2007
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