We present new Herschel Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA . observations of the ( 6,6 ) The correct spectroscopic notation should be J _ { K } = 6 _ { 6 } ^ { - } \leftarrow 6 _ { 6 } ^ { + } , but we follow here the notation commonly used in the ammonia literature . 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 , \sim 500 K , rotational temperatures characterizing the population of the highly-excited metastable H _ { 3 } O ^ { + } 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 \sim 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 .