Detection of extragalactic H3O+

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.