Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Galaxies appear simpler than expected   Galaxies are complex systems the evolution of which apparently results from
the interplay of dynamics, star formation, chemical enrichment, and feedback
from supernova explosions and supermassive black holes. The hierarchical theory
of galaxy formation holds that galaxies are assembled from smaller pieces,
through numerous mergers of cold dark matter. The properties of an individual
galaxy should be controlled by six independent parameters including mass,
angular-momentum, baryon-fraction, age and size, as well as by the accidents of
its recent haphazard merger history. Here we report that a sample of galaxies
that were first detected through their neutral hydrogen radio-frequency
emission, and are thus free of optical selection effects, shows five
independent correlations among six independent observables, despite having a
wide range of properties. This implies that the structure of these galaxies
must be controlled by a single parameter, although we cannot identify this
parameter from our dataset. Such a degree of organisation appears to be at odds
with hierarchical galaxy formation, a central tenet of the cold dark matter
paradigm in cosmology.

13
The categories of this paper are: quant-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
15
The categories of this paper are: nucl-ex
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga
16
The categories of this paper are: stat
21
The categories of this paper are: adap-org
11
The categories of this paper are: hep-lat
1
The categories of this paper are: cs
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th