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

We report the discovery of a compact group of galaxies, CGG-z5, at z similar to 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 '' x 300 (10 x 20 kpc(2)). All group members are HST /F435W and HST /F606W dropouts5 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-star/M-circle dot) approximate to 9:8, while the rest are low-mass satellites (log(M-star/M-circle dot) approximate to 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-star/M-circle dot) > 11) at z similar to 1. This implies that CGG-z5 could be a "proto-massive galaxy" captured during a short-lived phase of massive galaxy formation.