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Issue A&A
Volume 477, Number 1, January I 2008
Page(s) L5 - L8
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20078824



A&A 477, L5-L8 (2008)
DOI: 10.1051/0004-6361:20078824

Letter

Detection of extragalactic H $\mathsf{_3}$O $\mathsf{^+}$

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


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