Widespread Rotationally-Hot Hydronium Ion in the Galactic Interstellar
  Medium
  We present new 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 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 to 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.
