Variation in cumulative hydrogen column densities N(H I) and N(H) = N(H I) + 2N(H₂) vs. galactocentric radius. The solid blue curve for the total column density N(H) was derived by inverting the variation of E(J−K) with distance modulus in the 3D extinction data cube of Schultheis et al. (2014), as discussed in Sect. 2.3. The binned estimates for N(H) from H I and HCO⁺ or H I and CO assume N(HCO⁺)/N(H₂) =3×10^-9 or N(H₂) =2×10²⁰ cm^-2W_CO. The bins correspond to the ranges of galactocentric radius listed in Table 1.