Radio Selection of the Most Distant Galaxy Clusters

E. Daddi, S. Jin, V. Strazzullo, M. T. Sargent, T. Wang, C. Ferrari, E. Schinnerer, V. Smolčić, A. Calabró, R. Coogan, J. Delhaize, I. Delvecchio, D. Elbaz, R. Gobat, Q. Gu, D. Liu, M. Novak, F. Valentino

Research output: Contribution to journalArticlepeer-review

22 Scopus citations


We show that the most distant X-ray-detected cluster known to date, Cl J1001 at zspec = 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 S3 GHz > 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 zphot > 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.

Original languageEnglish
Article numberL31
JournalAstrophysical Journal Letters
Issue number2
StatePublished - 10 Sep 2017
Externally publishedYes


  • galaxies: clusters: general
  • galaxies: evolution
  • galaxies: formation
  • galaxies: high-redshift
  • radio continuum: galaxies


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