Mid-J CO Shock Tracing Observations of Infrared Dark Clouds III: SLED fitting

Abstract

Giant molecular clouds contain supersonic turbulence that can locally heat small fractions of gas to over 100 K. We run shock models for low-velocity, C-type shocks propagating into gas with densities between 10^3 and 10^5 cm^(-3) and find that CO lines are the most important cooling lines. Comparison to photodissociation region (PDR) models indicates that mid-J CO lines (J = 8-7 and higher) should be dominated by emission from shocked gas. In Papers I and II we presented CO J = 3-2, 8-7, and 9-8 observations toward four primarily quiescent clumps within infrared dark clouds. Here we fit PDR models to the combined spectral line energy distributions and show that the PDR models that best fit the low-J CO emission underpredict the mid-J CO emission by orders of magnitude, strongly hinting at a hot gas component within these clumps. The low-J CO data clearly show that the integrated intensities of both the CO J = 8-7 and 9-8 lines are anomalously high, such that the line ratio can be used to characterize the hot gas component. Shock models are reasonably consistent with the observed mid-J CO emission, with models with densities near 10^(4.5) cm^(-3) providing the best agreement. Where this mid-J CO is detected, the mean volume filling factor of the hot gas is 0.1%. Much of the observed mid-J CO emission, however, is also associated with known protostars and may be due to protostellar feedback.

28 pages, 18 figures, accepted by ApJ, grammatical and typesetting corrections

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• Photodissociation Region Diagnostics Across Galactic Environments
• 3D magnetic field morphology of the Perseus molecular cloud
• Lyα emission from galaxies in the Epoch of Reionization
• SHOCKFIND - An algorithm to identify magnetohydrodynamic shock waves in turbulent clouds
• Similar complex kinematics within two massive, filamentary infrared dark clouds
• Self-generated ultraviolet radiation in molecular shock waves II. CH+ and the interpretation of emission from shock ensembles
• Multicomponent kinematics in a massive filamentary IRDC
• The warm and dense Galaxy - tracing the formation of dense cloud structures out to the Galactic Center
• Infrared view of the multiphase ISM in NGC 253 II. Modelling the ionised and neutral atomic gas