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Abstract We briefly review three main applications of H α surveys in cosmology, namely: (1) the diffuse H α emission as a tracer of the free–free foreground that contaminates the fluctuations in the cosmic microwave background radiation; (2) the H α emission from galaxies as a measure of the formation rate of massive stars, both at low and high redshift; and (3) the diffuse H α emission from ionised clouds as a constraint on the local ionising background radiation.
D. Valls‐Gabaud (Thu,) studied this question.
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