Widespread Rotationally Hot Hydronium Ion in the Galactic Interstellar Medium
Journal article, 2014

We present new Herschel observations of the (6,6) and (9,9) inversion transitions of the hydronium ion toward Sagittarius B2(N) and W31C. Sensitive observations toward Sagittarius B2(N) show that the high, ~500 K, rotational temperatures characterizing the population of the highly excited metastable H3O+ rotational levels are present over a wide range of velocities corresponding to the Sagittarius B2 envelope, as well as the foreground gas clouds between the Sun and the source. Observations of the same lines toward W31C, a line of sight that does not intersect the Central Molecular Zone but instead traces quiescent gas in the Galactic disk, also imply a high rotational temperature of ~380 K, well in excess of the kinetic temperature of the diffuse Galactic interstellar medium. While it is plausible that some fraction of the molecular gas may be heated to such high temperatures in the active environment of the Galactic center, characterized by high X-ray and cosmic-ray fluxes, shocks, and high degree of turbulence, this is unlikely in the largely quiescent environment of the Galactic disk clouds. We suggest instead that the highly excited states of the hydronium ion are populated mainly by exoergic chemical formation processes and the temperature describing the rotational level population does not represent the physical temperature of the medium. The same arguments may be applicable to other symmetric top rotors, such as ammonia. This offers a simple explanation of the long-standing puzzle of the presence of a pervasive, hot molecular gas component in the central region of the Milky Way. Moreover, our observations suggest that this is a universal process not limited to the active environments associated with galactic nuclei.


molecular processes

galaxies: nuclei

interstellar matter: molecules


D. C. Lis

California Institute of Technology (Caltech)

Pierre and Marie Curie University (UPMC)

P. Schilke

University of Cologne

E. A. Bergin

University of Michigan

M. Gerin

Ecole Normale Superieure (ENS)

John H Black

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

C. Comito

University of Cologne

M. de Luca

Ecole Normale Superieure (ENS)

B. Godard

Ecole Normale Superieure (ENS)

R. Higgins

University of Cologne

Franck Le Petit

Ecole Normale Superieure (ENS)

J. C. Pearson

Jet Propulsion Laboratory, California Institute of Technology

E. W. Pellegrini

University of Toledo

T. G. Phillips

California Institute of Technology (Caltech)

S. Yu

Jet Propulsion Laboratory, California Institute of Technology

Astrophysical Journal

0004-637X (ISSN) 1538-4357 (eISSN)

Vol. 785 2 A135, pp. 1-9 135

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics


Basic sciences


Onsala Space Observatory



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