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We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observethe J_KaKc=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 thecentral molecular zone (CMZ) of our Galaxy. The map extends over approximately40 arcmin x 8 arcmin (~100x20 pc^2) along the Galactic plane with a linearresolution of 1.2 pc. The strongest of the three lines, the H_2CO (3_03-2_02)transition, is found to be widespread, and its emission shows a spatialdistribution similar to ammonia. The relative abundance of para-H_2CO is0.5-1.2 10^{-9}, which is consistent with results from lower frequency H_2COabsorption lines. Derived gas kinetic temperatures for individual molecularclouds range from 50 K to values in excess of 100 K. While a systematic trendtoward (decreasing) kinetic temperature versus (increasing) angular distancefrom 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 outsidethe dense clouds, the average kinetic temperature is 65+/-10 K. The hightemperatures of molecular clouds on large scales in the GC region may be drivenby turbulent energy dissipation and/or cosmic-rays instead of photons. Such anon-photon-driven thermal state of the molecular gas provides an excellenttemplate for the more distant vigorous starbursts found in ultraluminousinfrared galaxies (ULIRGs).

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