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-H_2CO 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 arcmin x 8 arcmin (~100x20 pc^2) along the Galactic plane with a linear
resolution 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 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 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).
