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