Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The origin of polar ring galaxies: evidence for galaxy formation by cold
  accretion   Polar ring galaxies are flattened stellar systems with an extended ring of
gas and stars rotating in a plane almost perpendicular to the central galaxy.
We show that their formation can occur naturally in a hierarchical universe
where most low mass galaxies are assembled through the accretion of cold gas
infalling along megaparsec scale filamentary structures. Within a large
cosmological hydrodynamical simulation we find a system that closely resembles
the classic polar ring galaxy NGC 4650A. How galaxies acquire their gas is a
major uncertainty in models of galaxy formation and recent theoretical work has
argued that cold accretion plays a major role. This idea is supported by our
numerical simulations and the fact that polar ring galaxies are typically low
mass systems.

20
The categories of this paper are: acc-phys
8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
36
The categories of this paper are: cmp-lg
32
The categories of this paper are: atom-ph
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
37
The categories of this paper are: supr-con
2
The categories of this paper are: math
4
The categories of this paper are: cond-mat
17
The categories of this paper are: q-fin
28
The categories of this paper are: alg-geom
24
The categories of this paper are: dg-ga
27
The categories of this paper are: comp-gas