Galactic dust clouds are shining in scattered Hα light

Bright emission nebulae, or H II regions, around hot stars are readily seen in H alpha light. However, the all-pervasive faint H alpha emission has only recently been detected and mapped over the whole sky. Mostly the H alpha emission observed along a line of sight is produced by ionized gas in situ. There are, however, cases where all or most of the H alpha radiation is due to scattering by electrons or dust particles that are illuminated by an H alpha-emitting source off the line of sight. Here we demonstrate that diffuse, translucent, and dark dust clouds at high Galactic latitudes are in many cases observed to have an excess of diffuse H alpha surface brightness; i.e., they are brighter than the surrounding sky. We show that the majority of this excess surface brightness can be understood as light scattered off the interstellar dust grains. The source of incident photons is the general Galactic H alpha background radiation impinging on the dust clouds from all over the sky.