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

TL;DR

This paper reports the discovery of a compact galaxy group at z~5 with JWST, providing insights into early galaxy assembly and predicting its evolution into a massive galaxy by z~1.

Contribution

First identification of a high-redshift compact galaxy group with JWST, linking observations to galaxy formation simulations and evolutionary pathways.

Findings

Group members are detected in JWST bands with robust photometric redshifts.

Simulations suggest the group merges into a massive galaxy by z=1.

The structure represents a proto-massive galaxy in formation.

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 $1.5"\times3"$ (10$\times$20~kpc$^2$). 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$(M_{*}/M_{\odot})\approx9.8$, while the rest are low-mass satellites (log$(M_{*}/M_{\odot})\approx8.4-9.2$). 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 \textsc{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$(M_{*}/M_{\odot})>11$) at z~1. This implies that CGG-z5 could be a "proto-massive galaxy" captured during a short-lived phase of massive galaxy formation.