Origin of the weak absorption near the confirmed protocluster

Determine whether the weak H I absorption features near the redshift of the confirmed protocluster originate from undetected faint member galaxies or from H I gas associated with the protocluster itself.

Background

The confirmed protocluster at z = 3.079 exhibits only weak H I absorption along the background-quasar sightline. Because the confirmed member galaxies have relatively large impact parameters from that sightline, the absorption could arise either from faint, undetected galaxies close to the sightline or from diffuse H I gas belonging to the protocluster.

The available spectra and sightline geometry do not provide enough information to distinguish these possibilities. Resolving the origin of the absorption is necessary for determining whether the protocluster contains a substantial reservoir of neutral hydrogen and for interpreting the relationship between galaxy overdensity and H I absorption.

References

Although undetected faint member galaxies, which may be located near the quasar sightline, can cause this absorption, we cannot currently distinguish whether these features originate from faint galaxies or the protocluster \ion{H}{i} gas.

— H I Absorbers as Beacons of Hidden Structure at $z \sim 3$: Multi-Component, Metal-Rich Absorption System near a Protocluster  (2609.04349 - Koretomo et al., 3 Sep 2026) in Section 3.3, “The H I gas in the protocluster” (subsection label: subsec:HIgas_PC)