Radial distribution of the [C ii] to ¹²CO emissivity ratio for Mask 3 (red circles) and Mask 4 (black boxes). We also show the radial distribution of the [C ii]/¹²CO ratio resulting from the combined emission from Mask 3 and 4 (blue triangles). The horizontal lines correspond to the average value for each mask region between 3−10 kpc. The horizontal lines are labeled with a range of pairs of the FUV field and H₂ volume densities (χ₀, n_H2), that reproduce their corresponding [C ii]/¹²CO ratio, as predicted by the Kaufman et al. (1999) PDR model. To facilitate the comparison with the PDR model, the ratios were calculated using [C ii] and ¹²CO emissivities expressed in units of erg s^-1 cm^-2 sr^-1 kpc^-1. In these units, typical uncertainties in the ratios are 180, 33, and 35 for Mask 3, Mask 4, and the combined emission from both mask regions, respectively.