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Astron. Astrophys. 345, 363-368 (1999)
3. Results
3.1. Detected cluster candidates
Using the various odd/even catalogs containing the objects
extracted from single frames (150 sec exposures), lists of cluster
candidates have been produced for each patch and band using the
cluster identification pipeline based on the matched-filter method
described in detail in Paper II. The cluster identification has
been applied to the I-band data from patch B, using the
same parameters to describe the cluster radial profile and luminosity
function ( kpc,
Mpc and
, ),
the same SExtractor detection parameters
( and
corresponding to the area of a
circle with radius ), and the same
selection criteria ( ,
and
) as given in Paper II. Over the
analyzed area of 1.1 square degrees, 19 new cluster candidates were
found, which gives a density of 17.2 cluster candidates per square
degree, consistent with the results discussed in Paper II and
with the value estimated by Postman et al. (1996). Out of the 19
detections 12 are "good" candidates, i.e. , those detected at
in at least one catalog, or at
in both I-band catalogs. In
addition, there are 7 candidates detected at
3 in only one catalog, nearly all at
large redshift ( ). In Paper II
the noise properties of the cluster-finding procedure were
investigated and the frequency of
noise peaks was estimated to be 4.2 per square degree. Using this
noise frequency it is found that the number of questionable candidates
is consistent with that expected for noise peaks, about 5 in the area
analyzed in Patch B.
The properties of the detected cluster candidates are summarized in
Table 1 which gives: in Column (1) the cluster id; in
Columns (2) and (3) the right ascension and declination (J2000);
in Column (4) the estimated redshift; in Column (5) the
measure of the cluster richness derived from the maximum likelihood
(see Paper II, Eq. (1)); Column (6) gives the number of
galaxies within a magnitude interval of 2 magnitudes delimited at the
bright end by the magnitude of third brightest cluster galaxy; in
Columns (7) and (8) the significance of the detections in the
even and odd catalogs, respectively; in Column (9) the
significance of the detection using the galaxy catalogs extracted from
the V images (see below); and in Column (10) other
identifications. The upper part of the table lists the "good"
candidates, while the remaining candidates are listed in the lower
part of the table. Note that the detection with the largest
significance is the cluster easily seen near the center of the patch.
This cluster is Abell S84 at (Abell
et al. 1989, Strubble & Rood 1987).
![[TABLE]](img29.gif)
Table 1. Preliminary cluster candidates for EIS Patch B.
For simultaneous display of the cluster candidates in all available
passbands a new facility for extracting image postage stamps from the
coadded images was implemented in the ESO Science Archive. Using this
facility all cluster candidates detected in I-band were
visually inspected and most were found to be promising. Even though
candidates in the lower part of the table are less conspicuous, most
do seem to be possible clusters. Out of the seven candidates listed in
the bottom part of the table, at least 4 lie in a region where the
limiting isophote varies significantly between the odd and even frames
which may explain the fact that they were detected only in the odd
catalogs. Note that 2 of them were detected in the V catalogs.
From this examination one also encounters cases where superposition of
clusters at significantly different redshifts may occur, as judged
from the available color information.
The cluster-finding pipeline was also used to detect cluster
candidates in the available band
catalogs for patches A and B. For this purpose the same radial
profile as for the band data, and the
same slope of the Schechter function were adopted, while the
characteristic magnitude was taken from Postman et al. (1996)
, corrected to the Johnson system
according to the transformation given by those authors. Furthermore
the limiting magnitude was chosen to fit the 80% completeness limit
for the band catalogs of
. Even/odd
band candidate catalogs were
produced, using the same peak finding criteria given above, but in
contrast to the I-band procedure no selection criteria on
detection significance, persistency or cluster richness (see
Paper II) were imposed.
These detections were cross-identified with the I-band ones
listed in Table 1, and those listed in Paper II for which
band data are available (16
candidates). The latter are listed in Table 2, which reproduces
part of Table 2 in Paper II, adding also the V-band
information. The matching between the I- and V-band
catalogs was done based on position only, using a search radius of
1 arcmin, centered on the nominal
band detection. The choice of
1 arcmin was dictated by the estimated uncertainty in the
position derived for the cluster candidates. It should be noted that
in most cases where a match is found, the estimated redshifts agree to
within . Only in 5 cases the
discrepancy was larger. The significance of the detections in the
V-band for the case of patch B cluster candidates is given
in Column (9) of Table 1 and for patch A in
Column (9) of Table 2. In case the candidate is detected
both in the even and in the odd catalogs, the listed V-band
significance is the highest of the two.
![[TABLE]](img35.gif)
Table 2. Cluster candidates for EIS Patch A that are found in the region where both I and V-band data are available. In addition to the information already presented in Table 2 of Paper II, here also the V-band detection significance is listed. The format of the table is the same as for Table 1.
In total there are 35 candidate clusters where color information is
available over a 2 square degree area. The regions where color
information is available are typical regions with respect to the data
quality as characterized by the seeing and limiting isophotes. The
distribution of estimated redshifts for this sample is shown in
Fig. 1. The median estimated redshift for the whole set presented in
this figure is . The shaded portion
of the histogram shows the distribution of estimated redshifts (as
obtained from the I-band detection) for the cluster candidates
also detected in V-band (combining patches A and B). Out
of 19 candidates with , 18
( 95%) are also detected in
V-band; for clusters at , 5
out of 16 are detected in V-band. This result is not surprising
since the ability to detect clusters varies with redshift, with the
redshift range of the candidates detected in V-band being
smaller than that of those detected in I-band. Using the rule
of thumb that the data should reach at least one magnitude fainter
than at the redshift of a given
cluster to allow for its detection, one can translate the galaxy
catalog limiting magnitudes adopted in the cluster search into
limiting redshifts for cluster detection. The I-band limit of
then translates into a limiting
redshift between (no-evolution
model) and (passive evolution
model). These values are in good agreement with the results presented
in Paper II, and by Postman et al. (1996), where cluster
detections are limited to . Using the
same argument, the V-band limit of
translates into a limiting redshift
between (no-evolution) and
(passive evolution). This argument
is consistent with the above findings, providing strong support for
the reality of the detected candidates. Note that the two matched
detections with estimated redshift
are among the cases where a significant discrepancy between redshifts
estimated from the V- and I-band data is present, with
the V-band estimate being significantly lower than the
I-band one.
![[FIGURE]](img47.gif) |
Fig. 1. The estimated redshift distribution for the 35 cluster candidates detected in regions where color information is available in patches A and B. The shaded region marks the distribution for candidates detected in both I- and V-band.
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The probability of detecting a cluster also depends on its
richness. Despite the small number statistics, nearby candidates not
detected in V data tend to be poor, with an estimated richness
close to the lower limit adopted for the inclusion of a candidate in
the catalog ( ). At high-redshifts,
only very rich clusters, probably with a large fraction of
ellipticals, are detected in the two passbands. There are two such
cases in the above table, but note that neither would have been
included as cluster candidates based on the V detection alone.
Their appearance on the images strongly suggests that both are likely
to be clusters at high redshifts. However, since some galaxies are
seen in the V images, either their matched filter redshifts are
overestimated or there are foreground concentrations leading to their
detection in the V data. Only spectroscopic follow-up will be
able to resolve such cases.
3.2. Color-magnitude diagrams
The availability of data in two passbands can, in principle,
provide an alternative way of confirming cluster candidates and their
estimated redshifts, based on the detection of the sequence of cluster
early-type galaxies in a CM diagram. As described in paper III a
color catalog was constructed for regions where multi-band data were
available. The color catalog was built by merging of the
and
band catalogs, associating objects
based on their extension (see Paper III for further details).
Using this color catalog a color-magnitude diagram, based on
magnitudes measured within a 5.3" aperture, was constructed for each
cluster candidate. This diagram includes all galaxies within a radius
of 0.5 h-1 Mpc (H0 = 75 km s-1/Mpc)
from the nominal cluster position that are detected in both bands.
Fig. 2 shows these diagrams for all cluster candidates with
. Also indicated in the plot are the
values of and the color of a typical
elliptical (no-evolution) at the estimated redshift of the cluster, as
derived from the matched filter. At low redshift, the sequence of
early-type galaxies is clearly visible, but at
the evidence for a CM relation is,
in most cases, less compelling. In no cases with
a CM sequence was visible, because
of the relatively shallow V-band data.
![[FIGURE]](img58.gif) |
Fig. 2. Color-magnitude diagrams observed for the cluster candidates where both V and I-band data are available and with estimated redshifts . For each cluster the name and matched filter estimated redshift are shown. The values of and the expected colors of typical ellipticals at the estimated redshifts are also indicated.
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Considering the combined patch A and B sample, one finds that
out of 35 clusters in the region of overlap of the V- and
I-band images, there are 11 with evidence for a CM relation,
with redshifts extending out to .
Furthermore, the redshift estimates based on color and the matched
filter seem to agree, in most cases, within 0.1. In addition there are
6 cases in that redshift range where there is a weaker suggestion of a
color sequence.
Comparison between the cases detected in both I and V
and the cases where a color sequence is found shows that in most cases
(16 out of 17) where the CM-diagram supports the detection, also the
candidate is detected in both bands. There are 4 cases where a
detection is found in both V and I-band and no color
sequence is seen. All of these cases have significances below 4, and
two of them correspond to the matched detections with
shown in Fig. 1. In one case
evidence for a color sequence is found but there is no detection in
the V-band. Altogether this shows that the V- and
I-band data provide a consistent picture, giving further
support to the reality of the detections based on I-band data
only.
© European Southern Observatory (ESO) 1999
Online publication: April 19, 1999
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