| Abstract
| - We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the JK AK c = 3 03 → 2 02, 3 22 → 2 21, and 3 21 → 2 20 transitions of para-H 2CO 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 × 20pc 2) along the Galactic plane with a linear resolution of 1.2pc. The strongest of the three lines, the H 2CO (3 03 → 2 02) transition, is found to be widespread, and its emission shows a spatial distribution similar to ammonia. The relative abundance of para-H 2CO is 0.5−1.2 × 10 -9, which is consistent with results from lower frequency H 2CO 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).
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