Substructure within the SSA22 protocluster at $z\approx3.09$
  We present the results of a densely sampled spectroscopic survey of the SSA22
protocluster at $z\approx 3.09$. Our sample with Keck/LRIS spectroscopy
includes 106 Ly$\alpha$ Emitters (LAEs) and 40 Lyman Break Galaxies (LBGs) at
$z=3.05-3.12$. These galaxies are contained within the $9'\times9'$ region in
which the protocluster was discovered, which also hosts the maximum galaxy
overdensity in the SSA22 region. The redshift histogram of our spectroscopic
sample reveals two distinct peaks, at $z=3.069$ (blue, 43 galaxies) and
$z=3.095$ (red, 103 galaxies). Furthermore, objects in the blue and red peaks
are segregated on the sky, with galaxies in the blue peak concentrating towards
the western half of the field. These results suggest that the blue and red
redshift peaks represent two distinct structures in physical space. Although
the double-peaked redshift histogram is traced in the same manner by LBGs and
LAEs, and brighter and fainter galaxies, we find that nine out of 10 X-ray AGNs
in SSA22, and all seven spectroscopically-confirmed giant Ly$\alpha$ "blobs,"
reside in the red peak. We combine our dataset with sparsely sampled
spectroscopy from the literature over a significantly wider area, finding
preliminary evidence that the double-peaked structure in redshift space extends
beyond the region of our dense spectroscopic sampling. In order to fully
characterize the three-dimensional structure, dynamics, and evolution of
large-scale structure in the SSA22 overdensity, we require the measurement of
large samples of LAE and LBG redshifts over a significantly wider area, as well
as detailed comparisons with cosmological simulations of massive cluster
formation.
