Volume 550, February 2013
|Number of page(s)||18|
|Section||Interstellar and circumstellar matter|
|Published online||07 February 2013|
The thermal state of molecular clouds in the Galactic center: evidence for non-photon-driven heating⋆,⋆⋆
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, 53121
2 Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
3 Astron. Dept., King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia
4 ESO, Karl-Schwarzschild Strasse 2, 85748 Garching bei München, Germany
5 Joint ALMA Observatory, Av. Alonso de Córdova 3107, Vitacura, Santiago, Chile
6 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
7 Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
8 National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA, 22903, USA
Accepted: 29 November 2012
We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the JKAKc = 303 → 202, 322 → 221, and 321 → 220 transitions of para-H2CO at 218GHz simultaneously to determine kinetic temperatures of the dense gas in the central molecular zone (CMZ) of our Galaxy. The map extends over approximately 40′ × 8′ (~100 × 20pc2) along the Galactic plane with a linear resolution of 1.2pc. The strongest of the three lines, the H2CO (303 → 202) transition, is found to be widespread, and its emission shows a spatial distribution similar to ammonia. The relative abundance of para-H2CO is 0.5−1.2 × 10-9, which is consistent with results from lower frequency H2CO absorption lines. Derived gas kinetic temperatures for individual molecular clouds range from 50K to values in excess of 100K. While a systematic trend toward (decreasing) kinetic temperature versus (increasing) angular distance from the Galactic center (GC) is not found, the clouds with highest temperature (Tkin> 100K) are all located near the nucleus. For the molecular gas outside the dense clouds, the average kinetic temperature is 65 ± 10K. The high temperatures of molecular clouds on large scales in the GC region may be driven by turbulent energy dissipation and/or cosmic-rays instead of photons. Such a non-photon-driven thermal state of the molecular gas provides an excellent template for the more distant vigorous starbursts found in ultraluminous infrared galaxies (ULIRGs).
Key words: Galaxy: center / ISM: clouds / ISM: molecules / radio lines: ISM
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2013
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