Correlation plot of the ammonia column density and H₂ column density. Straight lines indicate a high ammonia abundance χ_NH3 > 3 × 10^-7, an average value of  ~1.2 × 10^-7 and a low NH₃ abundance of 5 × 10^-8. As the contour plot illustration the same correlation is shown in the upper panel, the range of NH₃ column densities is divided into bins of 10¹⁵ cm^-2 and that of H₂ column densities into bins of 10²² cm^-2.