Abstract
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 twenty
proto-clusters have been observed at z > 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 seven starbursting, submillimeter-luminous
galaxies and five additional AGN within a volume of 4000 Mpc$^3$. As the
expected lifetime of both the luminous AGN and starburst phase of a galaxy is
~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
$\sim10^12M_ødot$ and we estimate that the cluster mass will exceed that
of the Coma supercluster at $z 0$. The filamentary structure is in line
with hierarchical growth simulations which predict that the peak of cluster
activity occurs rapidly at z > 2.
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