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Fig. 6

image

Line ratios (R = WCO (32)/WCO (21)) and optical depths (τCO (21)) in the conditions of NCO = 2.2 × 1017 cm−2 and δv = 3.5 km s−1 (estimated by median values in N131) as a function of kinetic temperature and volume density by RADEX modeling. The green contour indicates a region (or threshold) for a possible gas temperature-density distribution in a colliding flow at the onset of star formation from simulations in Clark et al. (2012).

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