Spectroscopic Confirmation of two Extremely Massive Protoclusters
  BOSS1244 and BOSS1542 at $z=2.24$
  We present spectroscopic confirmation of two new massive galaxy protoclusters
at $z=2.24\pm0.02$, BOSS1244 and BOSS1542, traced by groups of Coherently
Strong Ly$\alpha$ Absorption (CoSLA) systems imprinted in the absorption
spectra of a number of quasars from the SDSS III and identified as
overdensities of narrowband-selected H$\alpha$ emitters (HAEs). Using MMT/MMIRS
and LBT/LUCI near-infrared (NIR) spectroscopy, we confirm 46 and 36 HAEs in the
BOSS1244 and BOSS1542 fields, respectively. BOSS1244 displays a South-West (SW)
component at $z=2.230\pm0.002$ and another North-East (NE) component at
$z=2.246\pm0.001$ with the line-of-sight velocity dispersions of $405\pm202$ km
s$^{-1}$ and $377\pm99$ km s$^{-1}$, respectively. Interestingly, we find that
the SW region of BOSS1244 contains two substructures in redshift space, likely
merging to form a larger system. In contrast, BOSS1542 exhibits an extended
filamentary structure with a low velocity dispersion of $247\pm32$ km s$^{-1}$
at $z=2.241\pm0.001$, providing a direct confirmation of a large-scale cosmic
web in the early Universe. The galaxy overdensities $\delta_{\rm g}$ on the
scale of 15 cMpc are $22.9\pm4.9$, $10.9\pm2.5$, and $20.5\pm3.9$ for the
BOSS1244 SW, BOSS1244 NE, and BOSS1542 filament, respectively. They are the
most overdense galaxy protoclusters ($\delta_{\rm g}>20$) discovered to date at
$z>2$. These systems are expected to become virialized at $z\sim0$ with a total
mass of $M_{\rm SW}=(1.59\pm0.20)\times10^{15}$ $M_{\odot}$, $M_{\rm NE}
=(0.83\pm0.11)\times10^{15}$ $M_{\odot}$ and $M_{\rm
filament}=(1.42\pm0.18)\times10^{15}$ $M_{\odot}$, respectively. Together with
BOSS1441 described in Cai et al. (2017a), these extremely massive overdensities
at $z=2-3$ exhibit different morphologies, indicating that they are in
different assembly stages in the formation of early galaxy clusters.
