Graph showing the contours of P_th on the SFR map. The gray scale of SFR is recorded in the colorbar to the right. The contours of P_th ranges between 4 × 10⁵ (blue) and 2.4 × 10⁶ cm^-3 K (red) and have spacings equal to 4 × 10⁵ cm^-3 K. The crosses mark the locations of three pixels chosen from Fig. 5 (see text for more details). The four cyan diamonds mark the locations of four radio sources observed in Maddox et al. (2006).