Letter to the Editor

Massive galaxy formation caught in action at z ∼ 5 with JWST

¹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
e-mail: shuowen.jin@gmail.com
² DTU Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
e-mail: shuji@space.dtu.dk
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
⁵ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
⁶ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
⁷ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁸ Institut d’Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France

Received: 19 December 2022
Accepted: 18 January 2023

Abstract

We report the discovery of a compact group of galaxies, CGG-z5, at z ∼ 5.2 in the EGS field covered by the JWST/CEERS survey. CGG-z5 was selected as the highest overdensity of galaxies at z >  2 in recent JWST public surveys and it consists of six candidate members lying within a projected area of 1.5″ × 3″ (10×20 kpc²). All group members are HST/F435W and HST/F606W dropouts while securely detected in the JWST/NIRCam bands, yielding a narrow range of robust photometric redshifts 5.0 <  z <  5.3. The most massive galaxy in the group has a stellar mass log(M_*/M_⊙)≈9.8, while the rest are low-mass satellites (log(M_*/M_⊙)≈8.4–9.2). While several group members were already detected in the HST and IRAC bands, the low stellar masses and the compactness of the structure required the sensitivity and resolution of JWST for its identification. To assess the nature and evolutionary path of CGG-z5, we searched for similar compact structures in the EAGLE simulations and followed their evolution with time. We find that all the identified structures merge into a single galaxy by z = 3 and form a massive galaxy (log(M_*/M_⊙)> 11) at z ∼ 1. This implies that CGG-z5 could be a “proto-massive galaxy” captured during a short-lived phase of massive galaxy formation.