Comparison between Hα observations of H II regions and the DIG in the face-on galaxy M 81 by Greenawalt et al. (1998; middle row) and our simulation results (bottom row) of the ionization of the DIG by radiation escaping from the H II regions in the environment of the radiation sources. We find a reasonable qualitative agreement regarding both the denseand relatively intense H II regions around the star-forming regions on the left-hand side of the imageand the DIG on the right-hand side of the image. The simulation results presented in this image were computed for a combination of microclumping and macroclumping with a clumping factor of f_cl = 5 each. The brightness values in these images as well as those in Fig. 4 represent an emission measure range between 1 and 750 pc cm^-6 (logarithmicscaling). Observational images: ©AAS. Reproduced with permission.