ON THE ORIGINS OF THE HIGH-LATITUDE Hα BACKGROUND

The diffuse high-latitude Hα 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 Hα intensity in regions dominated by Galactic cirrus dust and much of the variance in the high-latitude Hα background are the result of scattering by interstellar dust of Hα photons originating elsewhere in the Galaxy. We provide an empirical relation, which relates the expected scattered Hα intensity to the IRAS 100 μm 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 Hα light is a suitable template for Galactic free–free foreground emission, is found to be in need of reexamination.

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