Shadow of a Colossus: A z=2.45 Galaxy Protocluster Detected in 3D Ly-a
  Forest Tomographic Mapping of the COSMOS Field
  Using moderate-resolution optical spectra from 58 background Lyman-break
galaxies and quasars at $z\sim 2.3-3$ within a $11.5'\times13.5'$ area of the
COSMOS field ($\sim 1200\,\mathrm{deg}^2$ projected area density or $\sim
2.4\,h^{-1}\,\mathrm{Mpc}$ mean transverse separation), we reconstruct a 3D
tomographic map of the foreground Ly$\alpha$ forest absorption at $2.2<z<2.5$
with an effective smoothing scale of
$\sigma_{3d}\approx3.5\,h^{-1}\,\mathrm{Mpc}$ comoving. Comparing with 61
coeval galaxies with spectroscopic redshifts in the same volume, we find that
the galaxy positions are clearly biased towards regions with enhanced IGM
absorption in the tomographic map. We find an extended IGM overdensity with
deep absorption troughs at $z=2.45$ associated with a recently-discovered
galaxy protocluster at the same redshift. Based on simulations matched to our
data, we estimate the enclosed dark matter mass within this IGM overdensity to
be $M_{\rm dm} (z=2.45) = (9\pm4)\times 10^{13}\,h^{-1}\,\mathrm{M_\odot}$, and
argue based on this mass and absorption strength that it will form at least one
$z\sim0$ galaxy cluster with $M(z=0) = (3\pm 2) \times
10^{14}\,h^{-1}\mathrm{M_\odot}$, although its elongated nature suggests that
it will likely collapse into two separate clusters. We also point out a compact
overdensity of six MOSDEF galaxies at $z=2.30$ within a $r\sim
1\,h^{-1}\,\mathrm{Mpc}$ radius and $\Delta z\sim 0.006$, which does not appear
to have a large associated IGM overdensity. These results demonstrate the
potential of Ly$\alpha$ forest tomography on larger volumes to study galaxy
properties as a function of environment, as well as revealing the large-scale
IGM overdensities associated with protoclusters and other features of
large-scale structure.
