Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: High-resolution images of five radio quasars at early cosmological
  epochs   Context: Until now, there have only been seven quasars at z>4.5 whose the
high-resolution radio structure had been studied in detail with Very Long
Baseline Interferometry (VLBI) imaging. Aims: We almost double the number of
VLBI-imaged quasars at these high redshifts with the aim of studying their
redshift-dependent structural and physical properties in a larger sample.
Methods: We observed five radio quasars (J0813+3508, J1146+4037, J1242+5422,
J1611+0844, and J1659+2101) at 4.5<z<5 with the European VLBI Network (EVN) at
1.6 GHz on 29 October 2008 and at 5 GHz on 22 October 2008. The angular
resolution achieved ranges from 1.5 to 25 milli-arcseconds (mas), depending on
the observing frequency, the position angle in the sky, and the source's
celestial position. Results: The sources are all somewhat extended on mas
scales, but compact enough to be detected at both frequencies. With one
exception of a flat-spectrum source (J1611+0844), their compact emission is
characterised by a steep radio spectrum. We found no evidence of
Doppler-boosted radio emission in the quasars in our sample. The radio
structure of one of them (J0813+3508) is extended to ~7", which corresponds to
43 kpc projected linear size. Many of the highest redshift compact radio
sources are likely to be young, evolving objects, far-away cousins of the
powerful gigahertz peaked-spectrum (GPS) and compact steep-spectrum (CSS)
sources that populate the Universe at lower redshifts.

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The categories of this paper are: math
1
The categories of this paper are: cs
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
30
The categories of this paper are: q-alg
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The categories of this paper are: hep-lat
21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
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The categories of this paper are: stat
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The categories of this paper are: hep-ph
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The categories of this paper are: alg-geom