Key points are not available for this paper at this time.
We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the J K A Kc = 3 03 2 02 , 3 22 2 21 , and 3 21 2 20 transitions of para-H 2 CO at 218 GHz 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 20 pc 2 ) along the Galactic plane with a linear resolution of 1.2 pc. The strongest of the three lines, the H 2 CO (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 2 CO is 0.5-1.2 10 -9 , which is consistent with results from lower frequency H 2 CO absorption lines. Derived gas kinetic temperatures for individual molecular clouds range from 50 K to values in excess of 100 K. 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 (T kin > 100 K) are all located near the nucleus. For the molecular gas outside the dense clouds, the average kinetic temperature is 65 10 K. 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).
Ao et al. (Tue,) studied this question.