Direct evidence for Ly$\alpha$ depletion in the protocluster core
  We have carried out a panoramic Ly$\alpha$ narrowband imaging with
Suprime-Cam on Subaru towards the known protocluster USS1558--003 at $z=2.53$.
Our previous narrowband imaging at near-infrared has identified multiple dense
groups of H$\alpha$ emitters (HAEs) within the protocluster. We have now
identified the large-scale structures across a $\sim$50 comoving Mpc scale
traced by Ly$\alpha$ emitters (LAEs) in which the protocluster traced by the
HAEs is embedded. On a smaller scale, however, there are remarkably few LAEs in
the regions of HAE overdensities. Moreover, the stacking analyses of the images
show that HAEs in higher-density regions show systematically lower escape
fractions of Ly$\alpha$ photons than those of HAEs in lower-density regions.
These phenomena may be driven by the extra depletion of Ly$\alpha$ emission
lines along our line of sight by more intervening cold
circumgalactic/intergalactic medium and/or dust existing in the dense core. We
also caution that all the past high-$z$ protocluster surveys using LAEs as the
tracers would have largely missed galaxies in the very dense cores of the
protoclusters where we would expect to see any early environmental effects.
