Connecting the growth of galaxies to the large-scale environment in a massive node of the Cosmic Web at z ∼ 3

A direct link between the large-scale environment and galaxy properties is very well established in the local Universe. However, very little is known about the role of the environment for galaxy growth before the peak of the cosmic star formation history at z > 3 due to the rarity of high-redshift, overdense structures. Using a combination of deep, multiwavelength observations, including MUSE, JWST, Chandra, HST, and ground-based imaging, we detected and studied the properties of a population of star-forming galaxies in the field of a hyperluminous quasar at z ≈ 3.25 associated with the giant Lyα nebula MQN01. We find that this region hosts one of the largest overdensities of galaxies discovered so far at z > 3, with ρ/ρ̄ = 53 ± 17 within 4 × 4 cMpc² and |Δv|≤1000 km s⁻¹ from the quasar, providing a unique laboratory for studying the link between overdense regions and galaxy properties at high redshift. Even in these rare overdense regions, galaxies form stars at a rate consistent with the main sequence at z ≈ 3, demonstrating that their star formation rate (SFR) is regulated by local properties correlated with their stellar mass rather than by their environment. However, the high-mass end of the stellar mass function is significantly elevated with respect to that of galaxies in the field at log(M_⋆/M_⊙)≳10.5, suggesting that massive galaxies in overdense regions build up their stellar mass earlier or more efficiently than in average regions of the Universe. Finally, the overdensity of color-selected Lyman break galaxies observed on larger scales, across ≈24 × 24 cMpc², is found to be aligned toward the structure traced by the spectroscopically confirmed galaxies identified with MUSE in the inner 4 × 4 cMpc², suggesting that this highly overdense region could extend further, up to a few tens of comoving megaparsecs.