Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The alignment of molecular cloud magnetic fields with the spiral arms in
  M33   The formation of molecular clouds, which serve as stellar nurseries in
galaxies, is poorly understood. A class of cloud formation models suggests that
a large-scale galactic magnetic field is irrelevant at the scale of individual
clouds, because the turbulence and rotation of a cloud may randomize the
orientation of its magnetic field. Alternatively, galactic fields could be
strong enough to impose their direction upon individual clouds, thereby
regulating cloud accumulation and fragmentation, and affecting the rate and
efficiency of star formation. Our location in the disk of the Galaxy makes an
assessment of the situation difficult. Here we report observations of the
magnetic field orientation of six giant molecular cloud complexes in the
nearby, almost face-on, galaxy M33. The fields are aligned with the spiral
arms, suggesting that the large-scale field in M33 anchors the clouds.

32
The categories of this paper are: atom-ph
30
The categories of this paper are: q-alg
29
The categories of this paper are: funct-an
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
10
The categories of this paper are: q-bio
26
The categories of this paper are: bayes-an
1
The categories of this paper are: cs
3
The categories of this paper are: physics
28
The categories of this paper are: alg-geom
15
The categories of this paper are: nucl-ex
17
The categories of this paper are: q-fin
37
The categories of this paper are: supr-con
13
The categories of this paper are: quant-ph
7
The categories of this paper are: hep-th
22
The categories of this paper are: chao-dyn