Atomic gas column density N_HI versus 160 μm optical depth τ₁₆₀ (left panel) and CO ACA (velocity-integrated) T_B versus τ₁₆₀ (right). Only regions with S/N ≥ 3 are shown (and included in the fits). The best-fit linear relations (Eq. (6)) for τ₁₆₀ as a function of N_HI are shown as heavy curves (in log space), and only consider the points with 10¹⁹  ≤  N_HI   ≤  10²¹ cm⁻² because of the need to consider the regions dominated by HI. The lighter curves show the fit to Eq. (5) as described in the text, and are used to determine . Also shown are the GMCs from Lee et al. (2018) assuming A_V/τ₁₆₀ = 2200 for consistency with their paper. The vertical line shows the equivalent of A_V = 1 assuming this conversion. The heavy (colored) curves in the right panel correspond to I_CO binned in log(τ₁₆₀), and the gray curve is the fit from Lombardi et al. (2006) for CO in the MW Pipe Nebula GMC versus A_V converted to τ₁₆₀ as above (see also the study Pineda et al. 2008, on the Perseus molecular cloud complex).