H I Absorbers as Beacons of Hidden Structure at z ~ 3: Multi-Component, Metal-Rich Absorption System near a Protocluster

H i gas traces the large-scale structure and provides the primary fuel for star formation. High-z protoclusters are ideal laboratories to study how H i gas is accreted and consumed during the formation of the most massive structures in the Universe. However, much remains unknown about the distribution and physical state of their H i gas. We examine a rare configuration in which a protocluster candidate is located in front of a quasar at z = 3.09. Our spectroscopic campaign confirms a protocluster at z = 3.079: however, no corresponding strong H i absorption is found in the background quasar spectrum. Instead, we serendipitously discover a prominent H i absorption feature at z ~ 3.01, offset by ~60 cMpc from the centre of the protocluster. Spanning an exceptionally broad velocity range of ~2000 km s−1 (~40 cMpc), this absorption is decoupled from the confirmed member galaxies. Detailed kinematic modelling reveals this absorption comprises five distinct components rather than a single cloud. Moreover, one of these components exhibits a super-solar metallicity ([O/H] = +1.19+0.91-0.78). We propose two physical scenarios for this unique system: (1) an additional, hidden massive protocluster along the line of sight, and/or (2) metal-rich outflows and metal-poor inflows driven by a single massive galaxy. The discovery highlights that while protoclusters are not universally associated with strong H i absorption, targeting the strong H i absorbers may serve as a beacon for uncovering massive, metal-rich protoclusters or complex gas kinematics in the early Universe.