Top, left panel: joint distribution of the (1-0) line integrated intensity as a function of the visual extinction. The number of sightlines falling in a given 2D bin of the distribution is color-coded using a linear scale to emphasize the bi-modal nature of the distribution. The white line shows the location of points that follows N(H₂) =X_CO . The red, orange, green, and blue vertical dashed lines show the visual extinction limits (1, 2, 6, and 15 mag, respectively) used in the masks of Fig. 6. Top, right panel: spatial distribution of the far UV illumination in units of the ISRF (Habing 1968). Bottom, left panel: ratio of the (1-0) line integrated intensity to the visual extinction. Bottom, right panel: joint distribution of this ratio as a function of the far UV illumination. The horizontal white line corresponds to the standard value of the X_CO factor. The blue, green, orange, and red vertical dashed lines show the far UV illumination limits (4, 10, 26, and 120, respectively), which corresponds to the dust temperature limits used in the masks of Fig. 8. The color scales of the two images show the same ranges as the axes of the bottom right joint distribution.