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Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891 reveal the presence of an H I halo extending up to at least 5 kpc from the plane. This halo gas appears to rotate 25 to 100 km s~1 more slowly than the gas in the plane. If this velocity di erence is due to the gradient in the gravitational potential, then it may serve to discriminate between disk and spheroidal mass models. The classic picture of a large outer are in the H I layer of NGC 891 may no longer be valid.
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Swaters et al. (1997) studied this question.
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