The HST-Hyperion survey: Environmental imprints on the stellar mass function at z  ∼  2.5

¹ Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
² Department of Physics and Astronomy, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
³ Gemini Observatory, NSF NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA
⁴ Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, 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 astronomico 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 Scuola Internazionale Superiore Studi Avanzati (SISSA), Physics Area, Via Bonomea 265, 34136 Trieste, Italy
¹⁰ Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France
¹¹ Department of Physics & Astronomy, College of Charleston, 58 Coming Street, Charleston, SC 29424, USA
¹² Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Fisica y Astronomia, Instituto de Astrofisica, Fernandez Concha 700, Las Condes, Santiago, RM, Chile
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 September 2025
Accepted: 30 January 2026

Abstract

Context. Not all galaxies at cosmic noon (2 ≲ z ≲ 3) evolve in the same way. Particularly, it remains unclear how and to what extent the local environment – especially the extreme overdensities of protoclusters – affects the stellar mass assembly of its constituent galaxies at high redshifts. The imprint of these early processes is encoded in the galaxy stellar-mass function (SMF); comparing SMFs across environments therefore reveals differences in evolutionary history.

Aims. We present the SMF of the Hyperion proto-supercluster at z ∼ 2.5, one of the largest and most massive protostructures in the early universe. This dataset yields the most statistically robust SMF of a single protostructure at z ≳ 2. By comparing the SMF of the overdense peaks within Hyperion to the coeval field, we begin to answer the question of how early, and how strongly, a dense environment tilts the balance in favor of massive galaxies

Methods. Given that Hyperion resides in the field of the Cosmic Evolution Survey (COSMOS), we combined the extensive COSMOS2020 photometric catalog with ground-based spectroscopy and new grism spectroscopy from the Hubble Space Telescope (HST). The structure of Hyperion is defined based on a three-dimensional overdensity map, allowing us to place galaxies into (i) the highly overdense peaks of Hyperion, (ii) the less-overdense outskirts of Hyperion, or (iii) a coeval field. We performed 100 Monte Carlo realizations of the data to propagate redshift and stellar mass uncertainties, refitting galaxy properties in each realization. After constructing SMFs for the outskirts and peaks of Hyperion, we normalized them to that of the field to highlight differences in the underlying shape of the SMFs.

Results. The overdense peaks of Hyperion host a striking excess of massive galaxies relative to the field: the number densities of log₁₀(M_*/M_⊙)∼11 galaxies are ∼10× higher than the coeval field, whereas log₁₀(M_*/M_⊙)∼9.5 galaxies are enhanced by only ∼3.5×. On the other hand, both the SMF of the outskirts of Hyperion and the SMF of Hyperion as a whole mirror the overall shape of the coeval field.

Conclusions. Environmental effects that govern stellar mass growth are already well established by z ∼ 2.5. The densest regions of Hyperion host galaxies that have already experienced accelerated stellar mass growth. Furthermore, this impact is largely masked in the total SMF of Hyperion, highlighting the necessity of deep spectroscopic surveys when uncovering environmental trends at high redshifts. These findings imply that high-redshift protostructures begin sculpting the high-mass end of the SMF well before the epoch when local clusters experience widespread quenching, and may provide the appropriate laboratories for producing the elevated star formation observed at cosmic noon.