Galactic Dust Clouds Are Shining in Scattered Hα Light

Bright emission nebulae, or HII regions, around hot stars are readily seen in Hα light. However, the all-pervasive faint Hα emission has only recently been detected and mapped over the whole sky. Mostly the Hα emission observed along a line of sight is produced by ionised gas in situ. There are, however, cases where all or most of the Hα radiation is due to scattering by electrons or dust particles which are illuminated by an Hα 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 Hα 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 Hα background radiation impinging on the dust clouds from all over the sky. Subject headings: ISM: general; clouds; dust, extinction; HII regions; radiative transfer; scattering