Galaxy overdensities around 3C radio galaxies and quasars at $1<z<2.5$
  revealed by Spitzer $3.6/4.5\mu$m and Pan-STARRS
  Luminous radio sources are thought to lie in galaxy clusters or
proto-clusters. The complete sample of 64 high-redshift 3C sources at $1<z<2.5$
has been mapped with the Spitzer Space Telescope. The IRAC $3.6\mu$m and
$4.5\mu$m 5-$\sigma$ detection limit of $4\mu$Jy (22.4 AB mag) allows us to
search for the brightest candidate cluster member galaxies associated with the
3C sources. To remove the contamination of foreground stars and galaxies along
the lines of sight toward the 3C sources we apply color cuts: removed sources
satisfy either the IRAC1/2 cut $[3.6] - [4.5] < -0.3$ or the Pan-STARRS/IRAC
cut $i - [4.5] < 0.5$ if detected by Pan-STARRS. For both selection methods,
about half of the 3C radio sources show significant overdensities ($>3\sigma$)
within 30" ($\sim$250 kpc) projected distance from the radio source compared to
the surrounding galaxy densities measured in the $50''-120''$ annulus. The
Pan-STARRS/IRAC cut reveals higher average overdensities than the IRAC1/2 cut
and also a decline of overdensities at $z>1.4$. To infer the nature of the
cluster members, we rerun the analysis using a stronger IRAC1/2 cut $[3.6] -
[4.5] < -0.1$ which removes $1<z<1.4$ passive ellipticals but not star-forming
galaxies. For the strong cut, the overdensities, on average, completely
disappear at $1<z<1.4$. We therefore suggest that the 4.5$\mu$m detected
cluster member galaxies are mainly passive ellipticals.
