To study the status and the evolution of clusters of galaxies at intermediate redshifts we built a sample of candidate clusters using radiogalaxies in the NRAO VLA Sky Survey as tracers of dense environments.
From the NVSS maps we extracted a catalogue of radio sources over an area of
550 square degrees, and made optical identifications with galaxies
brighter than
in the EDSGC Catalogue, resulting in a
sample of 1288 radiogalaxies (Zanichelli et al. 2001, Paper I).
In this paper we have presented the detection technique we applied to select
candidate groups and clusters associated to NVSS radio sources. The method is
based on the search of excesses in optical surface galaxy density nearby NVSS
radiogalaxies.
To keep low the probability of spurious radio-optical identifications, as well
as to preferentially select clusters at redshifts
,
we restricted
the cluster search to the 661 radiogalaxies having radio-optical distance
and magnitude
.
The search of regions having high optical galaxy density has been made using
the EDSGC galaxy catalogue, building matrices of galaxy counts down to
magnitude
.
This choice allows to find density excesses surrounding the faintest
radiogalaxies (identified down to
)
without introducing
significant incompleteness effects in the optical data.
Smoothing of galaxy counts has been done using a Gaussian filter with
.
The mode and standard deviation of smoothed galaxy counts
have been used to define a detection threshold for the surface density
excesses: we selected as cluster candidates those density excesses whose
centroid is within
from a radiogalaxy.
This search radius for candidate clusters associated to NVSS radio sources
corresponds to an Abell radius of a cluster at
.
By applying this cluster detection strategy to 661 radiogalaxies over
550 sq degrees at the South Galactic Pole, we obtained a sample of
171 cluster candidates. The estimated contamination level is about
.
Out of these 171 candidates, 76 correspond to already-known clusters, while
95 cluster candidates in our list do not have any known counterpart in the
literature and have been the subject of subsequent spectroscopic follow-up. The
full sample of radio-optically selected cluster candidates will be presented in
a following paper.
Multi Object Spectroscopy aimed to confirm the detection of clusters has been
successfully acquired at the 3.6 m ESO telescope for a subset of 12
candidates.
In 2 cases the radiogalaxy does not lie at the same redshift as any other
observed target, while 9 candidates have been confirmed as clusters of
galaxies in the redshift range
,
thus confirming that this
joint radio-optical cluster selection technique can be used as a powerful tool
for the detection of cluster candidates at intermediate redshifts.
For one additional candidate, the very low number of measured redshifts does
not allow any conclusion on the presence of a cluster surrounding the
radiogalaxy, and further observations are needed.
Velocity dispersions of the 9 spectroscopically confirmed clusters vary from
values typical of moderately rich clusters to those typical of groups or poor
clusters, thus strengthening the assumption that this technique is equally
efficient in selecting structures over a wide range of richness at different
redshifts.
If confirmed by future spectroscopic follow up, this last result could be of
great interest as this technique would offer the possibility to study the
properties of different environments, such as groups or rich clusters, in a
homogeneously selected cluster sample.
Acknowledgements
The authors acknowledge Marco Mignoli for his valuable help during the observation run.
Copyright ESO 2001