Best fit (χ² minimized) PDR models simulating the ¹²CO emission for Arp 299 A (left) and B+C (right) using three ISM phases. The red line is the sum of the three phase models, the black asterisks are the data points with error bars, and the blue, green, and orange lines represent the independent PDR models for each phases. The model density, temperature and column density are shown in the legend along with the relative contribution of each phase in terms of emission and column density.