# Direct evidence for Ly$\alpha$ depletion in the protocluster core

## TL;DR

This study provides direct observational evidence of Ly$ ext{alpha}$ depletion in the dense core regions of a high-redshift protocluster, revealing environmental effects on Ly$ ext{alpha}$ emission and implications for galaxy surveys.

## Contribution

It presents the first large-scale Ly$ ext{alpha}$ imaging of a protocluster, showing how dense environments suppress Ly$ ext{alpha}$ emission and highlighting potential biases in high-redshift galaxy surveys.

## Key findings

- Ly$ ext{alpha}$ emitters are scarce in dense protocluster cores.
- Higher-density regions show lower Ly$ ext{alpha}$ escape fractions.
- Dense environments may cause Ly$ ext{alpha}$ depletion via intervening medium or dust.

## Abstract

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.