Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Warm Gas and Ionizing Photons in the Local Group (ApJL version)   Several lines of argument suggest that a large fraction of the baryons in the
universe may be in the form of warm (T\sim 10^5-10^7 K) gas. In particular,
loose groups of galaxies may contain substantial reservoirs of such gas.
Observations of the cosmic microwave background by COBE place only weak
constraints on such an intragroup medium within the Local Group. The idea of a
Local Group corona dates back at least forty years (Kahn & Woltjer 1959). Here
we show that gas at T\sim 2-3 X 10^6 K (the approximate virial temperature of
the Local Group) -- extremely difficult to observe directly -- can in principle
radiate a large enough flux of ionizing photons to produce detectable H alpha
emission from embedded neutral clouds. However, additional constraints on the
corona -- the most stringent being pulsar dispersion measures towards the
Magellanic Clouds, and the timing mass -- rule out an intragroup medium whose
ionizing flux dominates over the cosmic background or the major Local Group
galaxies. A cosmologically significant coronal gas mass could remain invisible
to H alpha observations. More massive galaxy groups could contain extensive
coronae which are important for the baryon mass and produce a strong, local
ionizing flux.

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The categories of this paper are: math-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-alg
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The categories of this paper are: hep-ex
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The categories of this paper are: nlin
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The categories of this paper are: patt-sol
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The categories of this paper are: cond-mat
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The categories of this paper are: q-bio
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The categories of this paper are: hep-lat
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The categories of this paper are: mtrl-th
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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: eess
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The categories of this paper are: alg-geom
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The categories of this paper are: chao-dyn