2D histograms. (a) Visual extinction A_V as a function of the HCN/CO J =1–0 integrated intensity ratio (from maps at 120″ resolution). Dashed red, yellow, green, and blue horizontal lines are the visual extinction values 1, 4, 8, and 15 mag, respectively. Above each line, we show the percentage of the total HCN J = 1–0 luminosity that comes from the different A_V ranges. (b), (c), and (d) 2D histogram of the observed [C I] 492 GHz/CO J =1–0 line intensity ratio (in units of K km s⁻¹) as function of the observed HCN/CO J =1–0 line ratio for all A_V, for A_V < 3 mag, and for A_V > 3 mag. The dashed black curve shows the running median. Error bars show the standard deviation in the x-axis.