Line ratios (R = W_{CO (3–2)}/W_{CO (2–1)}) and optical depths (τ_{CO (2–1)}) in the conditions of N_CO = 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).