Integrated line intensity as a function of H₂ column density for the J = 1–0 transition of the four main CO isotopologs. For reference, each panel shows a linear relation that fits the average intensity of the highest column density bin (dashed line). Depending on the transition, the lower limit of the intensity scale (0.05 K km s⁻¹) approximately corresponds to one to three times the noise level in the integrated intensity.