A search for clusters at high redshift - II. A proto cluster around a radio galaxy at z=2.16

TL;DR

This study reports the discovery of a proto-cluster of galaxies around a radio galaxy at redshift 2.16, indicating a forming galaxy cluster with significant mass and substructure.

Contribution

It provides spectroscopic evidence of a proto-cluster at high redshift, identifying galaxy subgroups and estimating their masses, advancing understanding of early cluster formation.

Findings

14 galaxies and one QSO at similar redshift to the radio galaxy

Two galaxy subgroups with velocity dispersions of ~500 km/s and ~300 km/s

Total mass of the structure exceeds 10^14 solar masses

Abstract

VLT spectroscopic observations of Ly alpha--excess objects in the field of the clumpy radio galaxy 1138-262 at z=2.16 have led to the discovery of 14 galaxies and one QSO at approximately the same distance as the radio galaxy. All galaxies have redshifts in the range 2.16 \pm 0.02, centered around the redshift of the radio galaxy, and are within a projected physical distance of 1.5 Mpc from it. The velocity distribution suggests that there are two galaxy subgroups having velocity dispersions of ~500 km/s and ~300 km/s and a relative velocity of 1800 km/s. If these are virialized structures, the estimated dynamical masses for the subgroups are ~9 and ~3x10E13 solar masses respectively, implying a total mass for the structure of more than 10E14 solar masses. The new observations, together with previous results, suggest that the structure of galaxies around 1138-262 is likely to be a forming cluster.