Detection of extragalactic H3O+.pdfVIP

a r X i v : 0 7 1 1 .2 1 0 9 v 1 [ a s t r o - p h ] 1 4 N o v 2 0 0 7 Astronomy Astrophysics manuscript no. aa8824 c? ESO 2008 February 2, 2008 L????? ?? ??? E????? Detection of extragalactic H3O + F.F.S. van der Tak 1,2 , S. Aalto 3 , and R. Meijerink 4 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 M82 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 M82. 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 1. Introduction Starbursts and active galaxies often host large masses of molec- ular gas and dust in their inner kpc. Gravitational interaction or merging cause the funneling (via bars) of the gas towards the center. The average gas surface- and number den