Radio selection of the most distant galaxy clusters

TL;DR

This paper demonstrates that deep radio surveys can effectively identify the most distant galaxy clusters, like ClJ1001 at z=2.506, revealing their member galaxies' star formation and AGN activity, and highlighting future survey prospects.

Contribution

It shows that radio overdensities can be used to discover high-redshift galaxy clusters, with ClJ1001 as a case study, and discusses the potential for future wide-area radio surveys.

Findings

ClJ1001 is the strongest overdensity at z=2.506.

Radio sources in the cluster are mainly powered by star formation.

Future radio surveys can detect hundreds of high-redshift clusters.

Abstract

We show that the most distant X-ray detected cluster known to date, ClJ1001 at z=2.506, hosts a strong overdensity of radio sources. Six of them are individually detected (within 10") in deep 0.75" resolution VLA 3GHz imaging, with S(3GHz)>8uJy. Of the six, 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-square degree field using radio-detected 3GHz sources and looking for peaks in Sigma5 density maps. ClJ1001 is the strongest overdensity by far with >10sigma, with a simple z_phot>1.5 preselection. A cruder photometric rejection of z<1 radio foregrounds leaves ClJ1001 as the second strongest overdensity, while even using all radio sources ClJ1001 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.