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The current consensus on the formation and evolution of the brightest clustergalaxies is that their stellar mass forms early ($z \gtrsim 4$) in separategalaxies that then eventually assemble the main structure at late times ($z\lesssim 1$). However, advances in observational techniques have led to thediscovery of protoclusters out to $z \sim 7$, suggesting that the late-assemblypicture may not be fully complete. If these protoclusters assemble rapidly inthe early universe, they should form the brightest cluster galaxies muchearlier than suspected by the late-assembly picture. Using a combination ofobservationally constrained hydrodynamical and dark-matter-only simulations, weshow that the stellar assembly time of a sub-set of the brightest clustergalaxies occurs at high redshifts ($z > 3$) rather than at low redshifts ($z <1$), as is commonly thought. We find, using isolated non-cosmologicalhydrodynamical simulations, that highly overdense protoclusters assemble theirstellar mass into brightest cluster galaxies within $\sim 1$ $\mathrm{Gyr}$ ofevolution -- producing massive blue elliptical galaxies at high redshifts ($z\gtrsim 1.5$). We argue that there is a downsizing effect on the cluster scalewherein some of the brightest cluster galaxies in the cores of the most-massiveclusters assemble earlier than those in lower-mass clusters. In those clusterswith $z = 0$ virial mass $\geqslant 5\times10^{14}$ M$_\mathrm{\odot}$, we findthat $9.8$% have their cores assembly early, and a higher fraction of $16.4$%in those clusters above $10^{15}$ M$_\mathrm{\odot}$. The James Webb SpaceTelescope will be able to detect and confirm our prediction in the near future,and we discuss the implications to constraining the value of$\sigma_\mathrm{8}$.

Rapid early coeval star formation and assembly of the most massive galaxies in the universe