Mapping the Most Massive Overdensities through Hydrogen (MAMMOTH). II. Discovery of the Extremely Massive Overdensity BOSS1441 at z = 2.32

Cosmological simulations suggest a strong correlation between high optical-depth Lyα absorbers, which arise from the intergalactic medium, and 3D mass overdensities on scales of 10-30 comoving Mpc. By examining the absorption spectra of ∼80,000 QSO sight lines over a volume of 0.1 Gpc³ in the Sloan Digital Sky Survey III (SDSS-III), we have identified an extreme overdensity, BOSS1441, which contains a rare group of strong Lyα absorbers at . 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 Lyα emitters (LAEs) down to and reveal a large-scale structure of LAEs in this field. Our follow-up LBT observations have spectroscopically confirmed 20 galaxies in the density peak. We show that BOSS1441 has an LAE overdensity of 10.8 2.6 on a 15 comoving Mpc scale, which could collapse to a massive cluster with M 10¹⁵ at . This overdensity is among the most massive large-scale structures at discovered to date.