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

Understand

The current consensus on the formation and evolution of the brightest cluster galaxies is that their stellar mass forms early ($z \gtrsim 4$) in separate galaxies that then eventually assemble the main structure at late times ($z \lesssim 1$).

• However, advances in observational techniques have led to the discovery of protoclusters out to $z \sim 7$, 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 ($z < 1$), as is commonly thought.