Environments of Extended Radio Sources in the ATLBS Survey
  We present a study of the environments of extended radio sources in the
Australia Telescope Low Brightness Survey (ATLBS). The radio sources were
selected from the Extended Source Sample (ATLBS-ESS), which is a well defined
sample containing the most extended of radio sources in the ATLBS sky survey
regions. The environments were analyzed using 4-m CTIO Blanco telescope
observations carried out for ATLBS fields in the SDSS ${\rm r}^{\prime}$ band.
We have estimated the properties of the environments using smoothed density
maps derived from galaxy catalogs constructed using these optical imaging data.
The angular distribution of galaxy density relative to the axes of the radio
sources has been quantified by defining anisotropy parameters that are
estimated using a new method presented here. Examining the anisotropy
parameters for a sub-sample of extended double radio sources that includes all
sources with pronounced asymmetry in lobe extents, we find good evidence for
environmental anisotropy being the dominant cause for lobe asymmetry in that
higher galaxy density occurs almost always on the side of the shorter lobe, and
this validates the usefulness of the method proposed and adopted here. The
environmental anisotropy parameters have been used to examine and compare the
environments of FRI and FRII radio sources in two redshift regimes ($z<0.5$ and
$z>0.5$). Wide-angle tail sources and Head-tail sources lie in the most
overdense environments. The Head-tail source environments (for the HT sources
in our sample) display dipolar anisotropy in that higher galaxy density appears
to lie in the direction of the tails. Excluding the Head-tail and Wide-angle
tail sources, subsamples of FRI and FRII sources from the ATLBS survey appear
to lie in similar moderately overdense environments, with no evidence for
redshift evolution in the regimes studied herein.
