Large scale structures around radio galaxies at z~1.5

TL;DR

This study investigates the environments of two radio galaxies at z~1.5, revealing a significant overdensity of red galaxies and potential proto-cluster structures around one of them, using deep optical and near-infrared imaging.

Contribution

It introduces a method using BzK color criteria to identify cluster candidates and provides evidence of large-scale structures around a high-redshift radio galaxy.

Findings

Significant overdensity of red galaxies around 7C1756+6520

Presence of a red sequence indicating a proto-cluster

Overdensity of mid-IR selected AGN near 7C1756+6520

Abstract

We explore the environments of two radio galaxies at z~1.5, 7C1751+6809 and 7C1756+6520, using deep optical and near-infrared imaging. Our data cover 15'x15' fields around the radio galaxies. We develop and apply BzK color criteria to select cluster member candidates around the radio galaxies and find no evidence of an overdensity of red galaxies within 2Mpc of 7C1751+6809. In contrast, 7C1756+6520 shows a significant overdensity of red galaxies within 2Mpc of the radio galaxy, by a factor of 3.1 +/- 0.8 relative to the four MUSYC fields. At small separation (r<6''), this radio galaxy also has one z~1.4 evolved galaxy candidate, one z~1.4 star-forming galaxy candidate, and an AGN candidate (at indeterminate redshift). This is suggestive of several close-by companions. Several concentrations of red galaxies are also noticed in the full 7C1756+6520 field, forming a possible large-scale structure of evolved galaxies with a NW-SE orientation. We construct the color-magnitude diagram of red galaxies found near (r<2Mpc) 7C1756+6520, and find a clear red sequence that is truncated at Ks~21.5 (AB). We also find an overdensity of mid-IR selected AGN in the surroundings of 7C1756+6520. These results are suggestive of a proto-cluster at high redshift.