Average intensity in 2 kpc-wide radial rings in the plane of M 101 of TH derived with α_n = 0.92 (solid line) and with α_n= 0.82 and 1.02 (dotted lines), plotted against galacto-centric radius. The errors on the dotted lines are the same as on the solid line, but are not shown for clarity. The red line shows the radial distribution of the Hα emission (Hoopes et al. 2001), scaled to TH derived with α_n = 0.92 at the ring R = 16−18 kpc. Note the close corresponcence between the two distributions at R> 6 kpc. The discrepancy in the central part may be due to extinction in Hα and a possible overestimate of TH on H ii regions.