Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The HI Halo of NGC 891   Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891
reveal the presence of an HI halo extending up to at least 5 kpc from the
plane. This halo gas appears to rotate 25 to 100 km/s more slowly than the gas
in the plane. If this velocity difference 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 flare in the HI
layer of NGC 891 may no longer be valid. A correlation is seen between the
distributions of HI, Halpha and radio continuum emission, which supports, in
accordance with galactic fountain models, the picture of a substantial
disk-halo circulation related to the star formation activity in the disk of NGC
891. There is now also clear evidence for the presence of a rapidly rotating
(v_rot ~ 230 km/s) disk or ring of HI in the central part of NGC 891.

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The categories of this paper are: nucl-th
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The categories of this paper are: q-bio
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The categories of this paper are: stat
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The categories of this paper are: acc-phys
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The categories of this paper are: nlin
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The categories of this paper are: q-fin
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The categories of this paper are: dg-ga
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The categories of this paper are: bayes-an
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The categories of this paper are: quant-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: mtrl-th
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The categories of this paper are: cs
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The categories of this paper are: gr-qc
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The categories of this paper are: physics
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The categories of this paper are: supr-con
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The categories of this paper are: q-alg