Relation between the ¹²CO luminosity of a voxel and its H₂ mass at a galactocentric radius ﹩{R}_{\mathrm{gal}}﹩ = 5.6 kpc (top) and ﹩{R}_{\mathrm{gal}}﹩ = 11 kpc (middle), using the combined data sets (GRS+UMSB, EXFC 55–100 and 135–165). The relations are derived from voxels with ¹³CO emission >2﹩{\sigma }_{13}﹩ only. The slope of the relation is indicated in the legend, and corresponds to X_CO = ﹩1.9\times {10}^{20}﹩ cm⁻² (K km s⁻¹)⁻¹ s at ﹩{R}_{\mathrm{gal}}﹩ = 5.6 kpc , and X_CO = ﹩3.7\times {10}^{20}﹩ cm⁻² (K km s⁻¹)⁻¹ at ﹩{R}_{\mathrm{gal}}﹩ = 11 kpc. The bottom panel shows the X_CO factor as a function of Galactocentric radius ﹩{R}_{\mathrm{gal}}﹩.