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
clusters evolution. The candidates were found by studying the environment
around our newly selected sample of "beacons" low-luminosity (FRI) radio
galaxies in the COSMOS field. In this way we intend to use the fact that FRI 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 over-densities around our objects. Three out of the
five FRIs 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 4-sigma 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 FRI radio galaxies instead of the more
powerful FRII sources often used in the literature to date, is returning very
promising candidates.
