2017ApJ...846L..31D

We show that the most distant X-ray-detected cluster known to date, Cl J1001 at z_spec=2.506, hosts a strong overdensity of radio sources. Six of them are individually detected (within 10^′′ ) in deep 0′′ / . 75 resolution VLA 3 GHz imaging, with S_3GHz 8 µJy. Of the six, an active galactic nucleus (AGN) likely affects the radio emission in two galaxies, while star formation is the dominant source powering the remaining four. We searched for cluster candidates over the full COSMOS 2 deg² field using radio-detected 3 GHz sources and looking for peaks in Σ₅ density maps. Cl J1001 is the strongest overdensity by far with 10σ , with a simple z_phot 1.5 preselection. A cruder photometric rejection of z 1 radio foregrounds leaves Cl J1001 as the second strongest overdensity, while even using all radio sources Cl J1001 remains among the four strongest projected overdensities. We conclude that there are great prospects for future deep and wide-area radio surveys to discover large samples of the first generation of forming galaxy clusters. In these remarkable structures, widespread star formation and AGN activity of massive galaxy cluster members, residing within the inner cluster core, will ultimately lead to radio continuum as one of the most effective means for their identification, with detection rates expected in the ballpark of 0.1-1 per square degree at z 2.5. Samples of hundreds such high-redshift clusters could potentially constrain cosmological parameters and test cluster and galaxy formation models.