Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Metagalactic Ionizing Field in the Local Group   We discuss the sources which are likely to dominate the ionizing field
throughout the Local Group. In terms of the limiting flux to produce detectable
H-alpha emission (4-10 x 10**3 phot/cm**2/s), the four dominant galaxies (M31,
Galaxy, M33, LMC) have spheres of influence which occupy a small fraction
(<10%) of the Local Volume. There are at least two possible sources of
ionization whose influence could be far more pervasive: (i) a cosmic background
of ionizing photons; (ii) a pervasive warm plasma throughout the Local Group.
The COBE FIRAS sky temperature measurements permit a wide variety of plasmas
with detectable ionizing fields. It has been suggested (Blitz et al. 1996;
Spergel et al. 1996; Sembach et al. 1995, 1998) that a substantial fraction of
high velocity clouds are external to the Galaxy but within the Local Group.
Deep H-alpha detections are the crucial test of these claims and, indeed,
provide a test bed for the putative Local Group corona.

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The categories of this paper are: supr-con
14
The categories of this paper are: math-ph
10
The categories of this paper are: q-bio
9
The categories of this paper are: nlin
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The categories of this paper are: stat
11
The categories of this paper are: hep-lat
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The categories of this paper are: solv-int
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The categories of this paper are: dg-ga
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The categories of this paper are: chao-dyn
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The categories of this paper are: mtrl-th
13
The categories of this paper are: quant-ph
8
The categories of this paper are: hep-ex
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The categories of this paper are: comp-gas
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The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: econ