The thermal state of molecular clouds in the Galactic Center: evidence for non-photon driven heating

We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the J(KAKc) = 3(03) -> 2(02), 3(22) -> 2(21), and 3(21) -> 2(20) transitions of para-H2CO 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' x 8' (similar to 100 x 20 pc(2)) along the Galactic plane with a linear resolution of 1.2 pc. The strongest of the three lines, the H2CO (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-H2CO is 0.5 - 1.2 x 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 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).