Probing the effect of the protocluster environment on galaxy morphology at z = 2.23

¹ INAF – Osservatorio Astronomico di Padova Vicolo Osservatorio 5 35122 Padova, Italy
² Department of Astronomy, Tsinghua University Beijing 100084, China
³ Center for Fundamental Physics, School of Mechanics and Optoelectronic Physics, Anhui University of Science and Technology Huainan 232001, China
⁴ School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS) Beijing 100049, China
⁵ National Astronomical Observatories, Chinese Academy of Sciences Beijing 100101, China
⁶ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University Beijing 102206, China
⁷ Gemini Observatory, NSF NOIRLab 670 N. A’ohoku Place Hilo Hawai’i 96720, USA
⁸ Department of Physics and Astronomy, University of California, Davis One Shields Ave. Davis CA 95616, USA
⁹ European Southern Observatory Alonso de Córdova 3107 Vitacura Santiago de Chile, Chile
¹⁰ Department of Physics and Astronomy, Texas A&M University College Station TX 77843-4242, USA
¹¹ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University College Station TX 77843-4242, USA
¹² Institute for Astronomy, University of Hawai’i 2680 Woodlawn Drive Honolulu HI 96822, USA

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

Received: 3 March 2025
Accepted: 11 November 2025

Abstract

Context. As galaxies evolve in dense cluster and protocluster environments, they interact and quench their star formation. This gradually transforms the dominant galaxy population from star-forming galaxies to quiescent red galaxies. This transformation is identifiable by observing the galaxy colors and can also be seen in the morphological transformation of late-type galaxies into early-type galaxies, which creates the morphology-density relation observed when populations in clusters are compared to field galaxies at a given epoch. However, high-z (z > 2) galaxy morphology studies are hindered by the high angular resolution necessary to characterize the morphology.

Aims. We studied HST WFC3 F160W observations of protoclusters from the MAMMOTH survey (BOSS1244 and BOSS1542) at z ∼ 2.23 with populations of previously identified Hα emitters.

Methods. By measuring the Sérsic index of 151 Hα emitters, we searched for the early morphological transformation of high-z star-forming galaxies in these well-studied large nonvirialized protoclusters, which we think are the precursors of present-day galaxy clusters.

Results. The morphology of the populations of star-forming galaxies in protoclusters does not differ from that of their field counterparts. However, we also identified a population of clumpy potentially merging galaxies, which might increase the population of early-type galaxies within these structures. In BOSS1244, which has two previously identified massive quiescent galaxies including a brightest cluster galaxy (BCG), we additionally found an abundance of early-type galaxies near the BCG and two coeval high-z quasars.

Conclusions. Although the morphology of field and protocluster galaxies are very similar, the population of early-type star-forming galaxies surrounding the spectroscopically confirmed quiescent BCG in BOSS1244, which is not observed in BOSS1542, might indicate differences in the evolutionary state of these two coeval protoclusters. It might be a sign of an early forming cluster core in BOSS1244.