Abstract
It is known that the diffuse $H{\alpha}$ emission outside of bright H II regions not only are very extended, but also can occur in distinct patches or filaments far from H II regions, and the line ratios of [S II] ${\lambda}6716/H{\alpha}$ and [N II] ${\lambda}6583/H{\alpha}$ observed far from bright H II regions are generally higher than those in the H II regions. These observations have been regarded as evidence against the dust-scattering origin of the diffuse $H{\alpha}$ emission (including other optical lines), and the effect of dust scattering has been neglected in studies on the diffuse $H{\alpha}$ emission. However, as opposed to the previous contention, the expected dust-scattered $H{\alpha}$ halos surrounding H II regions are, in fact, in good agreement with the observed $H{\alpha}$ morphology. We find that the observed line ratios of [S II]/$H{\alpha}$, [N II]/$H{\alpha}$, and He I ${\lambda}5876/H{\alpha}$ in the diffuse ISM accord well with the dust-scattered halos around H II regions, which are photoionized by late O- and/or early B-type stars. We also demonstrate that the $H{\alpha}$ absorption feature in the underlying continuum from the dust-scattered starlight ("diffuse galactic light") and unresolved stars is able to substantially increase the [S II]/$H{\alpha}$ and [N II]/$H{\alpha}$ line ratios in the diffuse ISM.