Widespread Rotationally-Hot Hydronium Ion in the Galactic Interstellar Medium

Abstract

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.

Accepted for publication in The Astrophysical Journal, 10 pages, 4 figures, 3 tables

References in corpus (17)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• A computer program for fast non-LTE analysis of interstellar line spectra
• Molecular Gas at High Redshift
• Physical Conditions in Regions of Star Formation
• A Trigonometric Parallax of Sgr B2
• Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+
• The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions
• Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4
• Fading hard X-ray emission from the Galactic Centre molecular cloud Sgr B2
• Herschel observations of EXtra-Ordinary Sources: the present and future of spectral surveys with Herschel/HIFI
• Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220
• Strong absorption by interstellar hydrogen fluoride: Herschel/HIFI observations of the sight-line to G10.6-0.4 (W31C)
• Detection of Widespread Hot Ammonia in the Galactic Center
• Ammonia Thermometry of Star Forming Galaxies
• The kinetic temperature of a molecular cloud at redshift 0.9: Ammonia in the gravitational lens PKS1830-211
• Chemical Analysis of a Diffuse Cloud along a Line of Sight Toward W51: Molecular Fraction and Cosmic-Ray Ionization Rate
• H3+ Spectroscopy and the Ionization Rate of Molecular Hydrogen in the Central Few Parsecs of the Galaxy

Cited by in corpus (20)

• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Dense gas in the Galactic central molecular zone is warm and heated by turbulence
• Astrochemistry of dust, ice and gas: introduction and overview
• Interstellar Hydrides
• Ubiquitous argonium (ArH${}^{+}$) in the diffuse interstellar medium -- a molecular tracer of almost purely atomic gas
• Physical conditions in the central molecular zone inferred by H3+
• The Central 300 pc of the Galaxy probed by infrared spectra of H${}_3^{+}$ and CO: part I. Predominance of warm and diffuse gas and high H${}_2$ ionization rate
• A 1.3 cm line survey toward IRC +10216
• WISH VI. Constraints on UV and X-ray irradiation from a survey of hydrides in low- to high-mass YSOs
• A Centimeter-wave Study of Methanol and Ammonia Isotopologues in Sgr B2(N): Physical and chemical differentiation between two hot cores
• Accurate Prediction of the Ammonia Probes of a Variable Proton-to-Electron Mass Ratio
• Herschel observations of extraordinary sources: Full Herschel/HIFI molecular line survey of Sagittarius B2(M)
• Enhanced sensitivity to a possible variation of the proton-to-electron mass ratio in ammonia
• Accurate prediction of H${}_3$O${}^{+}$ and D${}_3$O${}^{+}$ sensitivity coefficients to probe a variable proton-to-electron mass ratio
• The physical and chemical structure of Sagittarius B2, VIII. Full molecular line survey of hot cores
• An accurate set of H${}_3$O${}^{+}-$ H${}_2$ collisional rate coefficients for non-LTE modelling of warm interstellar clouds
• Observations and Analysis of Absorption Lines including J = K Rotational Levels of CH3CN: The Envelope of Sagittarius B2(M)
• Cosmology, astrobiology, and the RNA world. Just add quintessential water
• Star Formation in the Galactic Center
• Spectral survey of the diffuse gas toward BL Lac in the Q band