The cluster environment of high redshift FRI radio galaxies
At variance with the powerful Fanaroff-Riley type II (FRII) radio sources, the great majority of low power radio sources (FRIs) are found in clusters, at least in the low redshift Universe. In order to prove that this also holds at high redshifts, and that these AGNs can be efficiently used as beacons for high-z clusters, we select a sample of bona fide Low Luminosity Radio Galaxies (LLRGs) at z~1-2 in the COSMOS field. We adopt our newly developed Poisson Probability Method (PPM) based on photometric redshifts and number counts to search for dense environments in the field of the LLRGs. We find evidence that about 70% of them reside in groups or clusters of galaxies. This is in excellent agreement with results obtained from low redshift studies. Our work indicates that the approach seems to be effective to search for clusters in a more unbiased way than conventional selection techniques (e.g. searches for concentrations of red galaxies, X-ray surveys or searches for the Sunyaev-Zeldovich effect). Our approach is very promising for large scale structure and AGN environment studies, especially at high redshift (i.e. z>~1.5), where the population of spectroscopically confirmed clusters is limited to a few cases and the properties of the cluster galaxy population in terms of galaxy morphologies, types, masses, colors, and star formation content are still debated.