Overdensity of i'-Dropout Galaxies in the Subaru Deep Field: A Candidate Protocluster at z ~ 6

TL;DR

This study identifies a significant overdensity of z~6 galaxies in the Subaru Deep Field, suggesting a candidate protocluster at high redshift that could evolve into a present-day galaxy cluster.

Contribution

The paper reports the discovery of a high-redshift galaxy overdensity and provides spectroscopic confirmation, proposing it as a candidate protocluster at z~6, which is a rare and valuable finding.

Findings

37 i'-dropouts clustered in a 21.6x21.6 Mpc^2 region

Four galaxies confirmed as Lyman-alpha emitters at z~6

Mass of the structure estimated at ~1.5x10^{15} solar masses

Abstract

We investigate the sky distribution of z ~ 6 Lyman break galaxies selected as i'-dropouts having i' - z' > 1.45 down to z' < 26.5 in the Subaru Deep Field (SDF). We discover 37 i'-dropouts clustered in a projected comoving 21.6 x 21.6 Mpc^2 region at z = 6, showing a local density excess. Carrying out follow-up spectroscopy, we identify four of them as Lyman-alpha emitters at z = 5.92, 6.01, 6.03 and 6.03 (spread over a distance of 46.6 Mpc). The number density of the cluster itself in SDF is ~ 2.2 x 10^{-7} Mpc^{-3}, smaller than those of protoclusters (i.e., forming galaxy clusters) at z ~ 2-5.7. Also, the structure shows ~4-21 times larger galaxy number density than those of z ~ 6 galaxies in a general field. It has a mass of M ~ 1.5^{+1.8}_{-0.5} x 10^{15}M_sun, comparable to those of z ~ 0-5 protoclusters. Since the contamination of our sample by interlopers is estimated to be quite low, 5.9%, most of the other unconfirmed i'-dropouts in the overdense region can be also z ~ 6 galaxies. Hence, it could be a candidate forming cluster at z ~ 6, representing a progenitor of galaxy clusters seen in the recent-day Universe.