Total hydrogen column density maps obtained via methods I and II. Left: high resolution N_H total gas column density map obtained from the Herschel maps of M33 at 18.2″ angular resolution using the β map from Tabatabaei et al. (2014) with method I. Right: total gas column density map N_H obtained from the Herschel SPIRE 250 μm map of M33 with method II at the same spatial resolution of 18.2″, indicated by the circle in the lower left corner. CO contours (as of Fig. 1) are overlaid in both maps. The white dashed ellipses mark roughly the regions we refer to as ‘inter-main spiral’ region or “outskirts”.