MApping the Most Massive Overdensities (MAMMOTH) II -- Discovery of an Extremely Massive Overdensity BOSS1441 at $z=2.32$

TL;DR

This paper reports the discovery of an extremely massive galaxy overdensity at redshift 2.32, identified through Ly$ extalpha$ absorption and confirmed by galaxy surveys, indicating a structure that could evolve into a massive galaxy cluster.

Contribution

It presents the identification and detailed analysis of one of the most massive large-scale structures at z~2, combining absorption spectra and galaxy surveys to characterize its mass and extent.

Findings

Discovered a large-scale overdensity with a 10.8$ imes$ LAE density enhancement.

Spectroscopically confirmed 19 galaxies within the overdensity.

The structure could evolve into a galaxy cluster with mass >10^{15} M_\odot.

Abstract

Cosmological simulations suggest a strong correlation between high optical-depth Ly$\alpha$ absorbers, which arise from the intergalactic medium (IGM), and 3-D mass overdensities on scales of $10-30$ $h^{-1}$ comoving Mpc. By examining the absorption spectra of $\sim$ 80,000 QSO sight-lines over a volume of 0.1 Gpc$^3$ in the Sloan Digital Sky Survey III (SDSS-III), we have identified an extreme overdensity, BOSS1441, which contains a rare group of strong Ly$\alpha$ absorbers at $z=2.32\pm 0.02$. This absorber group is associated with six QSOs at the same redshift on a 30 comoving Mpc scale. Using Mayall/MOSAIC narrowband and broadband imaging, we detect Ly$\alpha$ emitters (LAEs) down to $0.7\times L_{\rm{Ly\alpha}}^*$, and reveal a large-scale structure of Ly$\alpha$ emitters (LAEs) in this field. Our follow-up Large Binocular Telescope (LBT) observations have spectroscopically confirmed 19 galaxies in the density peak. We show that BOSS1441 has an LAE overdensity of $10.8\pm 2.6$ on a 15 comoving Mpc scale which could collapse to a massive cluster with $M\gtrsim10^{15}$ M$_\odot$ at $z\sim0$. This overdensity is among the most massive large-scale structures at $z\sim2$ discovered to date.