Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Radiogalaxies in the Sloan Digital Sky Survey: spectral
  index-environment correlations   We analyze optical and radio properties of radiogalaxies detected in the
Sloan Digital Sky Survey (SDSS). The sample of radio sources are selected from
the catalogue of Kimball & Ivezi\'c (2008) with flux densities at 325, 1400 and
4850 MHz, using WENSS, NVSS and GB6 radio surveys and from flux measurements at
74 MHz taken from VLA Low-frequency Sky Survey \citep{cohen}. We study
radiogalaxy spectral properties using radio colour-colour diagrams and find
that our sample follows a single power law from 74 to 4850 MHz. The spectral
index vs. spectroscopic redshift relation ($\alpha-z$) is not significant for
our sample of radio sources. We analyze a subsample of radio sources associated
with clusters of galaxies identified from the maxBCG catalogue and find that
about 40% of radio sources with ultra steep spectra (USS, $\alpha<-1$, where
$S_\nu \propto \nu^{\alpha}$) are associated with galaxy clusters or groups of
galaxies. We construct a Hubble diagram of USS radio sources in the optical $r$
band up to $z\sim0$.8 and compare our results with those for normal galaxies
selected from different optical surveys and find that USS radio sources are
around as luminous as the central galaxies in the maxBCG cluster sample and
typically more than 4 magnitudes brighter than normal galaxies at $z\sim0$.3.
We study correlations between spectral index, richness and luminosity of
clusters associated with radio sources. We find that USS at low redshift are
rare, most of them reside in regions of unusually high ambient density, such of
those found in rich cluster of galaxies. Our results also suggest that clusters
of galaxies associated with steeper than the average spectra have higher
richness counts and are populated by luminous galaxies in comparison with those
environments associated to radio sources with flatter than the average spectra.
A plausible explanation for our results is that radio emission is more pressure
confined in higher gas density environments such as those found in rich
clusters of galaxies and as a consequence radio lobes in rich galaxy clusters
will expand adiabatically and lose energy via synchrotron and inverse Compton
losses, resulting in a steeper radio spectra.

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The categories of this paper are: hep-th
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The categories of this paper are: dg-ga
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The categories of this paper are: supr-con
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The categories of this paper are: cs
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The categories of this paper are: bayes-an
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The categories of this paper are: alg-geom
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The categories of this paper are: q-fin
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The categories of this paper are: chem-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: atom-ph
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The categories of this paper are: nlin
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The categories of this paper are: acc-phys
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The categories of this paper are: math-ph
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The categories of this paper are: math
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-th