Early Quenching of Massive Protocluster Galaxies Around z=2.2 Radio Galaxies

Abstract: Radio galaxies are among the most massive galaxies in the high redshift universe and are known to often lie in protocluster environments. We have studied the fields of seven z = 2.2 radio galaxies with HAWK-I narrow-band and broad-band imaging in order to map out their environment using Halpha emitters (HAEs). The results are compared to the blank field HAE survey HiZELS. All of the radio galaxy fields are overdense in HAEs relative to a typical HiZELS field of the same area and four of the seven are richer than all except one of 65 essentially random HiZELS subfields of the same size. The star formation rates of the massive HAEs are lower than those necessary to have formed their stellar population in the preceding Gyr - indicating that these galaxies are likely to have formed the bulk of their stars at higher redshifts, and are starting to quench.