Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A 100-parsec elliptical and twisted ring of cold and dense molecular
  clouds revealed by Herschel around the Galactic Center   Thermal images of cold dust in the Central Molecular Zone of the Milky Way,
obtained with the far-infrared cameras on-board the Herschel satellite, reveal
a 3x10^7 solar masses ring of dense and cold clouds orbiting the Galactic
Center. Using a simple toy-model, an elliptical shape having semi-major axes of
100 and 60 parsecs is deduced. The major axis of this 100-pc ring is inclined
by about 40 degrees with respect to the plane-of-the-sky and is oriented
perpendicular to the major axes of the Galactic Bar. The 100-pc ring appears to
trace the system of stable x_2 orbits predicted for the barred Galactic
potential. Sgr A* is displaced with respect to the geometrical center of
symmetry of the ring. The ring is twisted and its morphology suggests a
flattening-ratio of 2 for the Galactic potential, which is in good agreement
with the bulge flattening ratio derived from the 2MASS data.

14
The categories of this paper are: math-ph
7
The categories of this paper are: hep-th
27
The categories of this paper are: comp-gas
12
The categories of this paper are: nucl-th
9
The categories of this paper are: nlin
4
The categories of this paper are: cond-mat
5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
1
The categories of this paper are: cs
15
The categories of this paper are: nucl-ex
0
The categories of this paper are: hep-ph
21
The categories of this paper are: adap-org
13
The categories of this paper are: quant-ph
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
16
The categories of this paper are: stat