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 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, multiwalength observations, including MUSE, JWST, Chandra, HST and ground-based imaging, we detect and study the properties of a population of star-forming galaxies in the field of a hyperluminous quasar at $z\approx3.25$ associated with the giant Ly$α$ nebula MQN01. We find that this region hosts one of the largest overdensity of galaxies found so far at $z>3$, with $ρ/\barρ=53\pm17$ within $4\times4\rm\,cMpc^2$ and $|Δv|<1000\rm\,km\,s^{-1}$ from the quasar, providing a unique laboratory to study the link between overdense regions and galaxy properties at high redshift. Even in these rare overdense regions, galaxies are forming stars at a rate consistent with the main sequence at $z\approx3$, demonstrating that their 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 $\log(M_\star/{M_\odot})\gtrsim10.5$, 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 $\approx24\times24\rm\,cMpc^2$, is found to be aligned toward the structure traced by the spectroscopically-confirmed galaxies identified with MUSE in the inner $4\times4\rm\,cMpc^2$, suggesting that this highly overdense region could further extend up to a few tens of comoving Mpc.