2013A&A...550A.135A

We used the Atacama Pathfinder Experiment (APEX) 12m telescope to observe the J_KA_Kc=3₀₃->2₀₂, 3₂₂->2₂₁, and 3₂₁->2₂₀ 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'x8' (∼100x20 pc2) along the Galactic plane with a linear resolution of 1.2 pc. 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 0.5-1.2x10^–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 (T_kin>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).