A Massive, Distant Proto-Cluster at Z=2.47 Caught in a Phase of Rapid Formation?

Numerical simulations of cosmological structure Formation show that the universe's most massive clusters, and the galaxies living in those clusters, assemble rapidly at early times (2.5 < z < 4). While more than 20 proto-clusters have been observed at z greater than or similar to 2 based on associations of 5-40 galaxies around rare sources, the observational evidence for rapid cluster Formation is weak. Here we report observations of an asymmetric filamentary structure at z = 2.47 containing 7 starbursting, submillimeter-luminous galaxies and 5 additional active galactic nuclei (AGNs) within a comoving volume of 15,000 Mpc(3). As the expected lifetime of both the luminous AGN and starburst phase of a galaxy is similar to 100 Myr, we conclude that these sources were likely triggered in rapid succession by environmental factors or, alternatively, the duration of these cosmologically rare phenomena is much longer than prior direct measurements suggest. The stellar mass already built up in the structure is similar to 10(12) M-circle dot and we estimate that the cluster mass will exceed that of the Coma supercluster at z similar to 0. The filamentary structure is in line with hierarchical growth simulations that predict that the peak of cluster activity occurs rapidly at z > 2.