In the following two sections we recall the properties of the radiogalaxy sample and of the optical galaxy catalogue that have been used during this search. For more details on the radio source catalogue and the definition of the radiogalaxy sample, refer to Paper I.
The radio source catalogue has been extracted from 31 maps of the 1.4 GHz
NRAO VLA Sky Survey (Condon et al. 1998) and consists of
pointlike and 2662 double radio sources down to a flux limit of 2.5 mJy beam-1
over an area of
550 sq degrees at the South Galactic Pole.
Optical identifications of NVSS radio sources have been made with galaxies
brighter than
in the EDSGC catalogue (Nichol et al.
2000) using a search radius of
,
i.e.
positional accuracy for the faintest sources.
The initial sample of optical counterparts consists of 1288 radiogalaxies,
926 of them having a pointlike radio morphology at the NVSS resolution of
.
As shown in Table 2 of Paper I, the contamination level due to spurious
identifications varies according to the radio morphological classification,
ranging from about
for the lists of optical counterparts of pointlike
radio sources and "close'' radio pairs (separation between components
), to about
for the list of optical counterparts of
"wide'' radio doubles (
).
In order to obtain a more reliable sample, the radiogalaxy data set used in the search of candidate clusters has been selected among these optical identifications on the basis of radio-optical distance and galaxy magnitude. The uncertainty in the optical identification sample is indeed the only source of contamination that can be limited when selecting cluster candidates by looking for excesses in surface galaxy density near the identified radiogalaxies. Other contamination terms - like the probability of detecting a candidate by chance coincidence of the radiogalaxy position with an optical density excess, or the possibility that the optical excess itself is intrinsically spurious, i.e. due to chance superpositions of galaxies along the line of sight - cannot in fact be reduced unless one knows the redshift distribution of the galaxies.
From the initial sample of optical counterparts we thus selected those
radiogalaxies having
.
This constraint introduces a
selection effect against faint sources in the radio sample, whose positional
uncertainty is typically
.
Furthermore, as our aim is to select candidate clusters at intermediate
redshifts, we discarded those radiogalaxies brighter than magnitude
.
In fact, considering the magnitude - redshift relation typical of
radiogalaxies obtained in the R band by Grueff & Vigotti (1977), and
using color indexes for elliptical galaxies given in Frei & Gunn
(1994), this cut in
magnitude corresponds to a redshift
lower limit of
.
With these constraints, the final radiogalaxy sample that has been taken into
account for the search of candidate clusters consists of 661 radiogalaxies,
and the mean, expected contamination level due to spurious optical
identifications has been lowered to about .
The Edinburgh-Durham Southern Galaxy Catalogue (EDSGC, Nichol et al.
2000) lists
galaxies over a contiguous area
of
1200 sq degrees at the South Galactic Pole.
About one half of this area is currently covered by our radiogalaxy sample and
has been considered for the search of cluster candidates.
The EDSGC has been obtained from COSMOS scans of IIIa-J ESO/SERC plates at
high galactic latitude (
). The automated
star-galaxy separation algorithm used for the EDSGC guarantees a completeness
>
and a stellar contamination <
down to magnitudes
.
Magnitudes have been calibrated via CCD sequences, providing a plate-to-plate
accuracy of
and an rms plate zero-point offset
of 0.05 magnitudes.
The EDSGC incompleteness starts to exceed the
only above
(Collins et al. 1992). When looking for
candidate clusters we thus decided to make optical galaxy counts down to the
magnitude limit
:
as the radiogalaxy sample reaches
,
this choice makes it possible to point out also those
regions of high galaxy surface density associated to the optically faintest
radio sources in our sample.
Copyright ESO 2001