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The Ubiquity of Coeval Starbursts in Massive Galaxy Cluster Progenitors

The Universe's largest galaxy clusters likely built the majority of their massive $>10^{11} M_\odot$ galaxies in simultaneous, short-lived bursts of activity well before virialization. This conclusion is reached from emerging datasets on $z>2$ 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, $\sim10^{4}$ comoving Mpc$^3$ at $z>2$, subtending areas $\sim$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 $2<z<3$ 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. Measurements of gas depletion times suggest that they are indeed short-lived on $\sim$100 Myr timescales, and the probability of finding a structure containing more than 8 such systems is $\sim$0.2\%, unless their `triggering' is correlated on very large spatial scales, $\sim$10 Mpc across. The volume density of starburst-rich proto-clusters is found to be comparable to all $>10^{15} M_\odot$ galaxy clusters in the nearby Universe, a factor of five larger than expected in some simulations. Some tension yet exists between measurements of their volume density of starburst-rich proto-clusters and the expectation that they are generated via short-lived episodes. 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. [abridged]