5 Summary and conclusions

The aim of our work was to test the validity of a model independent cluster finding algorithm, implemented to extract a statistically well defined sample of cluster candidates from photometrically calibrated DPOSS data (see Paper I for details). The advantages of a model independent approach are that i) the program does not assume any a priori knowledge about the clusters, and ii) it objectively looks for statistically meaningful overdensities in the galaxy density field. The main problem in validating any cluster finding algorithm is the lack of a suitable data set to use as a template, i.e., the lack of a region of the sky containing a large sample of clusters with well defined redshifts and properties. In the absence of such a data set, we adopted a photometric approach based on the use of the sequence defined in the color-magnitude diagrams of clusters by bright early-type galaxies as a diagnostic tool. We obtained deep multiband CCD photometry for a sample of 12 candidate clusters extracted from the DPOSS data, plus an additional sample of 8 X-ray clusters with known redshifts to be used as a template to calibrate the photometric redshift procedure. Results may be summarizied as follows: among the 12 clusters candidates, 10 are confirmed clusters, 1 is false and 1 is uncertain. The X-ray selected cluster sample was then used both to check the accuracy ( $\Delta z \simeq 0.01$) and to find the zero point (i.e., the average zero-redshift g-r color for elliptical galaxies in our system: (g-r)₀ =0.44) for the photometric redshift procedure. This procedure is being applied to a larger sample of clusters derived from both DPOSS calibration data and from other archive datasets. Future papers will deal with the analysis of a larger sample of clusters ($\sim$200) identified on both DPOSS and archive data and will focus on the derivation and analysis of luminosity functions (both individual and cumulative) and of radial number count profiles (Strazzullo 2001).

Figure 9: Color-magnitude diagrams after statistical background subtraction for the 12 candidate clusters with complete observations. The three upper right diagrams refer to not confirmed candidates (OV21_694, OV24_694, OV26_727).