We report the discovery of an extremely dense group of massive galaxies at the centre of the protocluster at z = 3.09 in the SSA22 field from near-infrared spectroscopy conducted with the Multi-Object InfraRed Camera and Spectrograph ( MOIRCS ) equipped on the Subaru Telecope .
The newly discovered group comprises seven galaxies confirmed at z _ { spec } \approx 3.09 within 180 kpc including five massive objects with the stellar masses larger than 10 ^ { 10.5 } ~ { } M _ { \odot } and is associated with a bright sub-mm source SSA22-AzTEC14 .
The dynamical mass of the group estimated from the line-of-sight velocity dispersion of the members is M _ { dyn } \sim 1.6 \pm 0.3 \times 10 ^ { 13 } ~ { } M _ { \odot } .
Such a dense group is expected to be very rare at high redshift as we found only a few comparable systems in large-volume cosmological simulations .
Such rare groups in the simulations are hosted in collapsed halos with M _ { vir } = 10 ^ { 13.4 } -10 ^ { 14.0 } ~ { } M _ { \odot } and evolve into the brightest cluster galaxies ( BCGs ) of the most massive clusters at present .
The observed AzTEC14 group at z = 3.09 is therefore very likely to be a proto-BCG in the multiple merger phase .
The observed total stellar mass of the group is 5.8 ^ { +5.1 } _ { -2.0 } \times 10 ^ { 11 } ~ { } M _ { \odot } .
It suggests that over half the stellar mass of its descendant had been formed by z = 3 .
Moreover , we identified over two members for each of the four Ly \alpha blobs ( LABs ) using our new spectroscopic data .
This verifies our previous argument that many of the LABs in the SSA22 protocluster associated with multiple developed stellar components .