Three candidate clusters of galaxies at redshift ~1.8: the "missing link" between protoclusters and local clusters?

TL;DR

This study identifies three potential galaxy clusters at redshift ~1.8 using low-luminosity radio galaxies as markers, providing insights into cluster evolution during an unexplored epoch.

Contribution

It introduces a novel method utilizing low-power FRI radio galaxies to efficiently find high-redshift galaxy clusters, expanding the tools for studying cluster formation.

Findings

Three candidate clusters at z~1.8 identified with 4-sigma significance.

Overdensities around FRI radio galaxies suggest rich cluster environments.

New method shows promise for discovering high-z clusters using low-power radio sources.

Abstract

We present three candidate clusters of galaxies at redshifts most likely between 1.7 and 2.0, which corresponds to a fundamentally unexplored epoch of clusters evolution. The candidates were found by studying the environment around our newly selected sample of "beacons" low-luminosity (FRI) radio galaxies in the COSMOS field. In this way we intend to use the fact that FRI at low z are almost invariably located in clusters of galaxies. We use the most accurate photometric redshifts available to date, derived by the COSMOS collaboration using photometry with a set of 30 filters, to look for three-dimensional space over-densities around our objects. Three out of the five FRIs in our sample which possess reliable photometric redshifts between z_phot = 1.7 and 2.0 display overdensities that together are statistically significant at the 4-sigma level, compared to field counts, arguing for the presence of rich clusters of galaxies in their Mpc environment. These first results show that the new method for finding high-z clusters we recently proposed, which makes use of low power FRI radio galaxies instead of the more powerful FRII sources often used in the literature to date, is returning very promising candidates.