Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A substructure inside spiral arms, and a mirror image across the
  Galactic Meridian   While the galactic density wave theory is over 50 years old and well known in
science, whether it fits our own Milky Way disk has been difficult to say. Here
we show a substructure inside the spiral arms. This substructure is reversing
with respect to the Galactic Meridian (longitude zero), and crosscuts of the
arms at negative longitudes appear as mirror images of crosscuts of the arms at
positive longitudes. Four lanes are delineated: mid-arm (extended 12CO gas at
mid arm, HI atoms), in-between offset by about 100 pc (synchrotron, radio
recombination lines), in between offset by about 200 pc (masers, colder dust),
and inner edge (hotter dust seen in Mid-IR and Near-IR).

17
The categories of this paper are: q-fin
22
The categories of this paper are: chao-dyn
18
The categories of this paper are: econ
21
The categories of this paper are: adap-org
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math
30
The categories of this paper are: q-alg
10
The categories of this paper are: q-bio
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The categories of this paper are: stat
32
The categories of this paper are: atom-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: nucl-th
4
The categories of this paper are: cond-mat
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The categories of this paper are: hep-ex
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The categories of this paper are: dg-ga
28
The categories of this paper are: alg-geom