2022ApJ...941..134H

The discovery and spectroscopic confirmation of Hyperion, a protosupercluster at z ∼ 2.47, provides an unprecedented opportunity to study distant galaxies in the context of their large-scale environment. We carry out deep narrowband imaging of a ≈1° × 1° region around Hyperion and select 157 Lyα emitters (LAEs). The inferred LAE overdensity is δ_g ≈ 40 within an effective volume of 30 × 20 × 15 cMpc³, consistent with the fact that Hyperion is composed of multiple protoclusters and will evolve into a supercluster with a total mass of M_tot ≈ 1.4 × 10¹⁵M_⊙ at z = 0. The distribution of LAEs closely mirrors that of known spectroscopic members, tracing the protocluster cores and extended filamentary arms connected to them, suggesting that they trace the same large-scale structure. By cross-correlating the LAE positions with H I tomography data, we find weak evidence that LAEs may be less abundant in the highest H I regions, perhaps because Lyα is suppressed in such regions. The Hyperion region hosts a large population of active galactic nuclei (AGNs) ≈ 12 times more abundant than that in the field. The prevalence of AGNs in protocluster regions hints at the possibility that they may be triggered by physical processes that occur more frequently in dense environments, such as galaxy mergers. Our study demonstrates LAEs as reliable markers of the largest cosmic structures. When combined with ongoing and upcoming imaging and spectroscopic surveys, wide-field narrowband imaging has the potential to advance our knowledge in the formation and evolution of cosmic structures and of their galaxy inhabitants.