Passive galaxies as tracers of cluster environments at z ~ 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 ~ 2 using massive passive galaxies as tracers. With a sample of ~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 large-scale 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-10¹³ M_⊙ range). Although this search approach is probably biased toward more evolved structures, it has the potential of selecting still rare, cluster-like environments close to their epoch of first appearance, enabling new investigations of the evolution of galaxies in the context of structure growth.