ON THE ORIGINS OF THE HIGH-LATITUDE Hα BACKGROUND

The diffuse high-latitude H alpha background is widely believed to be predominantly the result of in situ recombination of ionized hydrogen in the warm interstellar medium of the Galaxy. Instead, we show that both a substantial fraction of the diffuse high-latitude H alpha intensity in regions dominated by Galactic cirrus dust and much of the variance in the high-latitude H alpha background are the result of scattering by interstellar dust of H alpha photons originating elsewhere in the Galaxy. We provide an empirical relation, which relates the expected scattered H alpha intensity to the IRAS 100 mu m diffuse background intensity, applicable to about 81% of the entire sky. The assumption commonly made in reductions of cosmic microwave background observations, namely that the observed all-sky map of diffuse H alpha light is a suitable template for Galactic free-free foreground emission, is found to be in need of reexamination.