Three Candidate Clusters of Galaxies at Redshift ∼1.8: The “Missing Link” Between Protoclusters and Local Clusters?

We present three candidate clusters of galaxies at redshifts most likely between 1.7 and 2.0, which corresponds to a fundamentally unexplored epoch of cluster evolution. The candidates were found by studying the environment around our newly selected sample of "beacons" low-luminosity (FR I) radio galaxies in the COSMOS field. In this way, we intend to use the fact that FR Is at low z are almost invariably located in clusters of galaxies. We use the most accurate photometric redshifts available to date, derived by the COSMOS collaboration using photometry with a set of 30 filters, to look for three-dimensional space overdensities around our objects. Three out of the five FR Is in our sample which possess reliable photometric redshifts between z phot = 1.7 and 2.0 display overdensities that together are statistically significant at the 4σ level, compared to field counts, arguing for the presence of rich clusters of galaxies in their Mpc environment. These first results show that the new method for finding high- z clusters we recently proposed, which makes use of low-power FR I radio galaxies instead of the more powerful FR II sources often used in the literature to date, is returning very promising candidates.