Best-fit DL07 parameters plotted against normalized galactocentric distance. The upper left panel shows U_min; the upper right q_PAH; the lower left γ; and the lower right PDR fraction f_PDR. The underlying grayscales in each panel show the two-dimensional density distributions of the data. The upper left and right panels show the mean (solid curve) and quartiles (dashed curves) of U_min (upper left), q_PAH (upper right) and f_PDR (lower right). The (yellow) long-dashed curve in the upper left panel shows the relation for U_min =U₀ (T_dust/T₀)^(4+β) for U₀ = 0.93, T₀ = 22 K, and β = 1.8 as described in the text. As in previous figures, (red) circles correspond to 12 + log (O/H)> 8.4, (green) triangles to 8.1 <12 + log (O/H)≤ 8.4, and (blue) squares to 12 + log (O/H)≤ 8.1; filled symbols indicate R/R_opt< 0.8.