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We report the discovery of an extremely dense group of massive galaxies atthe centre of the protocluster at $z=3.09$ in the SSA22 field fromnear-infrared spectroscopy conducted with the Multi-Object InfraRed Camera andSpectrograph (MOIRCS) equipped on the Subaru Telecope. The newly discoveredgroup comprises seven galaxies confirmed at $z_{\rm spec}\approx3.09$ within180 kpc including five massive objects with the stellar masses larger than$10^{10.5}~M_{\odot}$ and is associated with a bright sub-mm sourceSSA22-AzTEC14. The dynamical mass of the group estimated from the line-of-sightvelocity dispersion of the members is $M_{\rmdyn}\sim1.6\pm0.3\times10^{13}~M_{\odot}$. Such a dense group is expected to bevery rare at high redshift as we found only a few comparable systems inlarge-volume cosmological simulations. Such rare groups in the simulations arehosted in collapsed halos with $M_{\rm vir}=10^{13.4}-10^{14.0}~M_{\odot}$ andevolve into the brightest cluster galaxies (BCGs) of the most massive clustersat present. The observed AzTEC14 group at $z=3.09$ is therefore very likely tobe a proto-BCG in the multiple merger phase. The observed total stellar mass ofthe group is $5.8^{+5.1}_{-2.0}\times10^{11}~M_{\odot}$. It suggests that overhalf the stellar mass of its descendant had been formed by $z=3$. Moreover, weidentified over two members for each of the four Ly$\alpha$ blobs (LABs) usingour new spectroscopic data. This verifies our previous argument that many ofthe LABs in the SSA22 protocluster associated with multiple developed stellarcomponents.

An extremely dense group of massive galaxies at the centre of the protocluster at z = 3.09 in the SSA22 field