Cluster candidates around low power radio-galaxies at z~1-2 in COSMOS
  We search for high redshift ($z\sim$1-2) galaxy clusters using low luminosity
radio galaxies (FR~I) as beacons and our newly developed Poisson Probability
Method (PPM) based on photometric redshift information and galaxy number
counts. We use a sample of 32 FR~Is within the Cosmic Evolution Survey (COSMOS)
field from Chiaberge et al. (2009) catalog. We derive a reliable subsample of
21 {\it bona fide} Low Luminosity Radio Galaxies (LLRGs) and a subsample of 11
High Luminosity Radio Galaxies (HLRGs), on the basis of photometric redshift
information and NRAO VLA Sky Survey (NVSS) radio fluxes. The LLRGs are selected
to have 1.4~GHz rest frame luminosities lower than the fiducial FR~I/FR~II
divide. This also allows us to estimate the comoving space density of sources
with $L_{1.4}\simeq 10^{32.3}\,\hbox{erg}\,\hbox{s}^{-1}\,\hbox{Hz}^{-1}$ at
$z\simeq 1.1$, which strengthens the case for a strong cosmological evolution
of these sources. In the fields of the LLRGs and HLRGs we find evidence that 14
and 8 of them reside in rich groups or galaxy clusters, respectively. Thus,
overdensities are found around $\sim70\%$ of the FR~Is, independently of the
considered subsample. This rate is in agreement with the fraction found for low
redshift FR~Is and it is significantly higher than that of FR~IIs at all
redshifts. Although our method is primarily introduced for the COSMOS survey,
it may be applied to both present and future wide field surveys such as SDSS
Stripe 82, LSST, and Euclid. Furthermore, cluster candidates found with our
method are excellent targets for next generation space telescopes such as JWST.
