Passive galaxies as tracers of cluster environments at z similar to 2

Even 10 billion years ago, the cores of the first galaxy clusters are often found to host a characteristic population of massive galaxies with already suppressed star formation. Here we search for distant cluster candidates at z similar to 2 using massive passive galaxies as tracers. With a sample of similar to 40 spectroscopically confirmed passive galaxies at 1.3 < z < 2.1, we tuned photometric redshifts of several thousand passive sources in the 2 sq. deg COSMOS field. This allowed us to map their density in redshift slices, probing the largescale structure in the COSMOS field as traced by passive sources. We report here on the three strongest passive galaxy overdensities that we identify in the range 1.5 < z < 2.5. While the actual nature of these concentrations still needs to be confirmed, we discuss their identification procedure and the arguments supporting them as candidate galaxy clusters (probably in the mid-10(13) M-circle dot range). Although this search approach is probably biased toward more evolved structures, it has the potential of selecting still rare, clusterlike environments close to their epoch of first appearance, enabling new investigations of the evolution of galaxies in the context of structure growth.