On the nature of the EIS candidate clusters: Confirmation of $z\lesssim$ 0.6 candidates

Abstract

We use publicly available V -band imaging data from the wide-angle surveys conducted by the ESO Imaging Survey project (EIS) to further investigate the nature of the EIS galaxy cluster candidates. These were originally identified by applying a matched-filter algorithm which used positional and photometric data of the galaxy sample extracted from the I -band survey images. In this paper, we apply the same technique to the galaxy sample extracted from V -band data and compare the new cluster detections with the original ones. We find that ~75% of the low-redshift cluster candidates ($z\lesssim0.6$) are detected in both passbands and their estimated redshifts show good agreement with the scatter in the redshift differences being consistent with the estimated errors of the method. For the robust I -band detections the matching frequency approaches ~85% . We also use the available $(V-I)$ color to search for the of early-type galaxies observed in rich clusters over a broad range of redshifts. This is done by searching for a simultaneous overdensity in the three-dimensional color-projected distance space. We find significant overdensities for ~75% of the robust candidates with $z_I\lesssim0.6$. We find good agreement between the characteristic color associated to the detected red sequence and that predicted by passive evolution galaxy models for ellipticals at the redshift estimated by the matched-filter. The results presented in this paper show the usefulness of color data, even of two-band data, to both tentatively confirm cluster candidates and to select possible cluster members for spectroscopic observations. Based on the present results, we estimate that ~150 EIS clusters with $z_I\lesssim0.6$ are real, making it one of the largest samples of galaxy clusters in this redshift range currently available in the southern hemisphere.