The HST-Hyperion Survey: Companion fraction and overdensity in a z ∼ 2.5 proto-supercluster

¹ Institute for Astronomy, University of Hawai‘i 2680 Woodlawn Drive Honolulu HI 96822, USA
² Gemini Observatory, NSF NOIRLab 670 N Aohoku Pl Hilo HI 96720, USA
³ Department of Physics and Astronomy, University of California, Davis One Shields Ave Davis CA 95616, USA
⁴ Department of Physics and Astronomy, Texas A&M University College Station TX 77843-4242, USA
⁵ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University College Station TX 77843-4242, USA
⁶ INAF– Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Piero Gobetti 93/3 40129 Bologna, Italy
⁷ INAF – Osservatorio di Padova Vicolo Osservatorio 5 35122 Padova, Italy
⁸ Department of Astronomy & Astrophysics, University of California, San Diego 9500 Gilman Dr La Jolla CA 92093, USA
⁹ INAF – IASF Milano Via A. Corti 12 20133 Milano, Italy
¹⁰ Italian National Institute for Astrophysics – Trieste Observatory Via GB Tiepolo 11 34143 Trieste, Italy
¹¹ IFPU-Institute for Fundamental Physics of the Universe Via Beirut 2 34014 Trieste, Italy
¹² Scuola Internazionale Superiore Studi Avanzati (SISSA), Physics Area Via Bonomea 265 34136 Trieste, Italy
¹³ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA
¹⁴ Observatoire de Sauverny Chemin Pegasi 51 1290 Versoix, Switzerland
¹⁵ Tianjin Astrophysics Center, Tianjin Normal University Binshuixidao 393 300384 Tianjin, China
¹⁶ Instituto de Astrofísica de Andalucía (CSIC) Apartado 3004 18080 Granada, Spain

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Received: 6 March 2025
Accepted: 6 November 2025

Abstract

We present a study of the galaxy merger and interaction activity within the Hyperion Proto-supercluster at z ∼ 2.5 in an effort to assess the occurrence of galaxy mergers and interactions in contrast to the coeval field and their impact on the buildup of stellar mass in high-density environments at higher redshifts. For this work, we utilized data from the Charting Cluster Construction with VUDS and ORELSE Survey (C3VO) along with extensive spectroscopic and photometric datasets available for the COSMOS field – including the HST-Hyperion Survey. To evaluate potential merger and interaction activity, we measured the fraction of galaxies with close kinematic companions (f_ckc) both within Hyperion and the coeval field by means of a Monte Carlo (MC) methodology developed in this work that probabilistically employs our entire combined spectroscopic and photometric dataset. We validated our f_ckc MC methodology on a simulated lightcone built from the GAlaxy Evolution and Assembly (GAEA) semi-analytic model, and we determined correction factors that account for the underlying spectroscopic sampling rate of our dataset. We find that galaxies in Hyperion have close kinematic companions ≳2.5× more than galaxies in the field and measure a corrected f_ckc = 59⁺⁹₋₁₀% for Hyperion and a corrected f_ckc = 23^+1.7_−1.8% for the surrounding field; a ≳3σ difference. The enhancement in f_ckc likely correlates to an enhancement in the merger and interaction activity within the high-density environment of Hyperion and matches the trend seen in other structures. The rate of merger and interactions within the field implied from our field f_ckc measurement is well aligned with values measured from other observations in similar redshift ranges. The enhanced f_ckc measured within Hyperion suggests that merger and interaction activity play an important role in the mass growth of galaxies in denser environments at higher redshifts.