Optically selected galaxy clusters at high redshift

People involved at OAB: Bardelli, Zucca.

A sample of a few hundred galaxy cluster candidates has been extracted from the wide angle multicolor ESO Imaging Survey (EIS), using a matched filter algorithm in the $I\,$ band. The estimated redshift range of these candidates has a high redshift tail reaching $z\sim 1.3$. This sample will allow to determine the structural parameters and the galaxy population characteristics of clusters of different richness in a wide range of redshifts (Da Costa et al. 2001).

A great effort has been undertaken in order to have a spectroscopic confirmation of a subsample of high redshift EIS clusters. In particular, we confirmed three clusters at $z=0.81, 1.14, 1.30$ (Benoist et al. 2002; Joergensen, Bardelli, Zucca et al. in prep.). It is particularly important that the two systems at $z>1$ are the most distant clusters identified so far by their optical properties alone. The cluster at $z=1.30$ coincides remarkably well with the location of a firm X-ray detection ($>5\sigma$) in a $\sim 80$ ksec XMM-Newton image (Neumann, ... Bardelli, Zucca et al. 2003). Moreover, we detected in the same image another X-ray emission from a serendipitously found concentration of infrared galaxies at an estimated redshift of $z=1.7$. The two emissions are consistent with a point-like profile and therefore are likely to be associated with cluster AGN. However, we cannot exclude that the hot gas emission of the clusters is extremely peaked, as expected for proto-clusters.

This work is carried out in collaboration with L. da Costa and S. Arnouts (ESO), C. Benoist (Nice Obs.), L. Olsen and H. Jørgensen (Copenhagen Obs.), A. Biviano and M. Ramella (INAF-Trieste Obs.), M. Scodeggio (IASF-CNR, Milan), D. Neumann and M. Arnaud (CEA/CEN, Saclay).