CHAMP+observations of warm gas in M 17 SW

Context. Sub-millimeter and Far-IR observations have shown the presence of a significant amount of warm (few hundred K) and dense (n(H 2 ) 10 4 cm -3 ) gas in sources ranging from active star-forming regions to the vicinity of the Galactic center. Since the main cooling lines of the gas phase are important tracers of the interstellar medium in Galactic and extragalactic sources, proper and detailed understanding of their emission and the ambient conditions of the emitting gas, is necessary for a robust interpretation of the observations. Aims. With high resolution (7 -9 ) maps (3 3 pc 2 ) of mid-J molecular lines we aim to probe the physical conditions and spatial distribution of the warm (50 to several hundred K) and dense gas (n(H 2 ) > 10 5 cm -3 ) across the interface region of the nearly edge-on M 17 SW nebula. Methods. We have used the dual color multiple pixel receiver CHAMP + on the APEX telescope to obtain a 5. 3 4. 7 map of the J = 6 5 and J = 7 6 transitions of 12 CO, the 13 CO J = 6 5 line, and the 3 P 2 3 P 1 370 m fine-structure transition of C I in M 17 SW. LTE and non-LTE radiative transfer models are used to constrain the ambient conditions. Results. The warm gas extends up to a distance of 2.2 pc from the M 17 SW ridge. The 13 CO J = 6 5 and C I 370 m lines have a narrower spatial extent of about 1.3 pc along a strip line at PA = 63 . The structure and distribution of the C I 3 P 2 3 P 1 370 m map indicate that its emission arises from the interclump medium with densities on the order of 10 3 cm -3 . Conclusions. The warmest gas is located along the ridge of the cloud, close to the ionization front. An LTE approximation indicates that the excitation temperature of the embedded clumps reaches 120 K. The non-LTE model suggests that the kinetic temperature at four selected positions cannot exceed 230 K in clumps of a density of n(H 2 ) 5 10 5 cm -3 and that the warm (T k > 100 K) and dense (n(H 2 ) 10 4 cm -3 ) gas traced by the mid-J 12 CO lines represents just about 2% of the bulk of the molecular gas. The clump volume-filling factor ranges between 0.04 and 0.11 at these positions.