Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observations of the Extended Distribution of Ionized Hydrogen in the
  Plane of M31   We have used the Wisconsin H-Alpha Mapper (WHAM) to observe the spatially
extended distribution of ionized hydrogen in M31 beyond the stellar disk. We
obtained five sets of observations, centered near the photometric major axis of
M31, that extend from the center of the galaxy to just off the edge of the
southwestern HI disk. Beyond the bright stellar disk, but within the HI disk,
weak H-alpha is detected with an intensity I(H-alpha) = 0.05 (+0.01 / -0.02)
Rayleighs. Since M31 is inclined 77 degrees with respect to the line of sight,
this implies that the ambient intergalactic ionizing flux onto each side of M31
is Phi_0 <= 1.6 x 10^4 photons cm^-2 s^-1. Just beyond the outer boundary of
the HI disk we find no significant detection of H-alpha and place an upper
limit I(H-alpha) <= 0.019 Rayleighs.

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The categories of this paper are: cond-mat
37
The categories of this paper are: supr-con
1
The categories of this paper are: cs
24
The categories of this paper are: dg-ga
12
The categories of this paper are: nucl-th
27
The categories of this paper are: comp-gas
33
The categories of this paper are: plasm-ph
18
The categories of this paper are: econ
28
The categories of this paper are: alg-geom
30
The categories of this paper are: q-alg
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The categories of this paper are: gr-qc
36
The categories of this paper are: cmp-lg
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The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
17
The categories of this paper are: q-fin
3
The categories of this paper are: physics