4 The validation of the candidate cluster sample

In Fig. 9 we show the early-type sequences obtained (after subtracting the background) for the 12 candidate clusters in our sample. We confirm 9 of the 12 candidates as true clusters, one is a fortuitous detection of a cluster at $z~\sim~0.4$, one is a false detection and the last is undecidable on the basis of the available data.

The better definition of the early-type sequences observed in the DPOSS confirmed clusters sample with respect to the X-ray sample is likely due to the different specific properties of the two samples: at a given z, our optically selected clusters are on the average richer and more centrally concentrated then the X-ray selected ones.

For the cluster OV27_694, the early-type sequence is less defined due to the overlap of two independent clusters/groups along the line of sight. The case of OV21_694 merits special attention. As a visual inspection of the corresponding POSS-II F plate shows, the marginal detection of an early type sequence (Figs. 5 and 9) seems to be due to the chance alignment of a distant cluster (at redshift $z \sim 0.4$) with a rich galaxy field. The existence of such a foreground rich field has therefore triggered the search algorithm. As far as OV24_694 and OV26_727 are concerned, the marginal evidence for an early-type sequence does not correspond to a defined overdensity in the number count radial profiles (using galaxies in the strip centered on the mean g-r color). The case of OV26_727 is a false cluster detection, since it has a low S/N ratio and low isophotal richness (see Table 1; it may be a group). Visual inspection of the POSS-II plate shows that OV24_694 lies in a crowded field rich with galaxies; from the sky diagram (Fig. 5d) it is also evident that a large fraction of these foreground galaxies have the same color. Thus, this field could be part of a larger loose cluster or a cluster in a region with variable background.

 

 

Table 3: New redshift estimate. (1): The faintness of the galaxies, as observed both on CCD and on plate suggests that the candidate cluster is far, as puts forward by the redness of the color-magnitude relation; (2): the color-magnitude shows a large scatter, suggesting that this cluster is possibly contaminated by a foreground group.

OBJ	z est.	z est. min	z est. max
17_778	0.195	0.169	0.209
1_778	0.234	0.219	0.247
1_799	0.314	0.299	0.326
21_694(1)	0.488	0.466	0.511
24_694	0.282	0.271	0.295
26_727	0.216	0.204	0.23
27_694(2)	0.204	0.183	0.218
44_778	0.243	0.228	0.255
5_778	0.189	0.159	0.205
64_781	0.139	0.110	0.159
6_725	0.218	0.205	0.232
72_781	0.219	0.207	0.234