2016ApJ...824...36C

The universe's largest galaxy clusters likely built the majority of their massive >10¹¹ M _☉ galaxies in simultaneous, short-lived bursts of activity well before virialization. This conclusion is reached based on emerging data sets for z\gt 2 proto-clusters and the characteristics of their member galaxies, in particular, rare starbursts and ultraluminous active galactic nuclei (AGN). The most challenging observational hurdle in identifying such structures is their very large volumes, ∼10⁴ comoving Mpc³ at z\gt 2 , subtending areas of approximately half a degree on the sky. Thus, the contrast afforded by an overabundance of very rare galaxies in comparison to the background can more easily distinguish overdense structures from the surrounding, normal density field. Five 2\lesssim z\lesssim 3 proto-clusters from the literature are discussed in detail and are found to contain up to 12 dusty starbursts or luminous AGN galaxies each, a phenomenon that is unlikely to occur by chance even in overdense environments. These are contrasted with three higher-redshift ( 4\lesssim z\lesssim 5.5 ) dusty star-forming galaxy (DSFG) groups, whose evolutionary fate is less clear. Measurements of DSFGs' gas depletion times suggest that they are indeed short-lived on ∼100 Myr timescales, and accordingly the probability of finding a structure containing more than 8 such systems is ∼0.2%, unless their "triggering" is correlated on very large spatial scales, ∼10 Mpc across. The volume density of DSFG-rich proto-clusters is found to be comparable to all of the >10¹⁵ M _☉ galaxy clusters in the nearby universe, which is a factor of five larger than expected in some simulations. Some tension still exists between measurements of the volume density of DSFG-rich proto-clusters and the expectation that they are generated via short-lived episodes, as the latter suggests that only a fraction ( \lt 1{2} ) of all proto-clusters should be rich with DSFGs. However, improved observations of proto-clusters over large regions of sky will certainly shed more light on the assembly of galaxy clusters, and whether or not they build their galaxies through episodic bursts as suggested here.