a) Emission of the CO J = 2 → 1 plotted against the 1.3 mm continuum emission. The Pearson coefficient of correlation of the data, P_c, is indicated in the lower right-hand corner. No extrapolated data are used here, only observations are reported and used to do the linear regression (green full line, equation indicated in the lower right-hand corner). b) Same as a), for the CO J = 3 → 2 emission against the 0.8 mm continuum emission. Fluxes are rescaled at 100 pc.