Monday

The current consensus on the formation and evolution of the brightest cluster galaxies is that their stellar mass forms early (z≳4) in separate galaxies that then eventually assemble the main structure at late times (z≲1). However, advances in observational techniques have led to the discovery of protoclusters out to z∼7, suggesting that the late-assembly picture may not be fully complete. If these protoclusters assemble rapidly in the early universe, they should form the brightest cluster galaxies much earlier than suspected by the late-assembly picture. Using a combination of observationally constrained hydrodynamical and dark-matter-only simulations, we show that the stellar assembly time of a sub-set of the brightest cluster galaxies occurs at high redshifts (z>3) rather than at low redshifts (z1), as is commonly thought. We find, using isolated non-cosmological hydrodynamical simulations, that highly overdense protoclusters assemble their stellar mass into brightest cluster galaxies within ∼1 Gyr of evolution -- producing massive blue elliptical galaxies at high redshifts (z≳1.5). We argue that there is a downsizing effect on the cluster scale wherein some of the brightest cluster galaxies in the cores of the most-massive clusters assemble earlier than those in lower-mass clusters. In those clusters with z=0 virial mass ⩾5e14 M⊙, we find that 9.8% have their cores assembly early, and a higher fraction of 16.4% in those clusters above 1015 M⊙. The James Webb Space Telescope will be able to detect and confirm our prediction in the near future, and we discuss the implications to constraining the value of σ8.