# Imaging of diffuse HI absorption structure in the SSA22 proto-cluster region at z = 3.1

## TL;DR

This study maps diffuse HI absorption in the SSA22 proto-cluster at z=3.1, revealing extended HI gas distribution and its relation to galaxy environments and Lya emission properties.

## Contribution

Introduces a new photometric measure of HI Lya absorption and maps its distribution in a proto-cluster, highlighting large-scale diffuse gas and galaxy-CGM interactions.

## Key findings

- HI absorption is enhanced in the SSA22 proto-cluster compared to control fields.
- Diffuse HI gas extends over more than 50 comoving Mpc.
- HI absorption near galaxies correlates with Lya emission strength.

## Abstract

Using galaxies as background light sources to map intervening Lya absorption is a novel approach to study the interplay among galaxies, the circum-galactic medium (CGM), and the intergalactic medium (IGM). Introducing a new measure of z = 3.1$ HI Lya absorption relative to the cosmic mean, Delta_NB497, estimated from photometric data of star-forming galaxies at 3.3 < z < 3.5, we have made two-dimensional Delta_NB497 maps in the z = 3.1 SSA22 proto-cluster region and two control fields (SXDS and GOODS-N fields) with a spatial resolution of ~ 5 comoving Mpc. The Delta_NB497 measurements in the SSA22 field are systematically larger than those in the control fields, and this HI absorption enhancement extends more than 50 comoving Mpc. The field-averaged (i.e., ~50 comoving Mpc scale) Delta_NB497 and the overdensity of Lya emitters (LAEs) seem to be correlated, while there is no clear dependency of the Delta_NB497 on the local LAE overdensity in a few comoving Mpc scale. These results suggest that diffuse HI gas spreads out in/around the SSA22 proto-cluster. We have also found an enhancement of Delta_NB497 at a projected distance < 100 physical kpc from the nearest z = 3.1 galaxies at least in the SSA22 field, which is probably due to HI gas associated with the CGM of individual galaxies. The HI absorption enhancement in the CGM-scale tends to be weaker around galaxies with stronger Lya emission, which suggests that the Lya escape fraction from galaxies depends on hydrogen neutrality in the CGM.