Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A large population of galaxies 9 to 12 billion years back in the history
  of the Universe   To understand the evolution of galaxies, we need to know as accurately as
possible how many galaxies were present in the Universe at different epochs.
Galaxies in the young Universe have hitherto mainly been identified using their
expected optical colours, but this leaves open the possibility that a
significant population remains undetected because their colours are the result
of a complex mix of stars, gas, dust or active galactic nuclei. Here we report
the results of a flux-limited I-band survey of galaxies at look-back times of 9
to 12 billion years. We find 970 galaxies with spectroscopic redshifts between
1.4 and 5. This population is 1.6 to 6.2 times larger than previous estimates,
with the difference increasing towards brighter magnitudes. Strong ultraviolet
continua (in the rest frame of the galaxies) indicate vigorous star formation
rates of more than 10 to 100 solar masses per year. As a consequence, the
cosmic star formation rate is higher than previously measured at redshifts of 3
to 4.

16
The categories of this paper are: stat
33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
14
The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
10
The categories of this paper are: q-bio
30
The categories of this paper are: q-alg
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
21
The categories of this paper are: adap-org
4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
20
The categories of this paper are: acc-phys
12
The categories of this paper are: nucl-th
2
The categories of this paper are: math