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) on the Subaru Telecope. The newly discovered group comprises seven galaxies confirmed at zspec ≈ 3.09 within 180 kpc, including fivemassive objectswith the stellarmasses larger than 1010.5 M⊙ and is associated with a bright submillimetre source SSA22-AzTEC14. The dynamical mass of the group estimated from the line-of-sight velocity dispersion of the members isMdyn ~ 1.6 ± 0.3 × 1013M⊙. Such a dense group is expected to be very rare at high redshift, as we have found only a few comparable systems in large-volume cosmological simulations. Such rare groups in simulations are hosted in collapsed haloes with Mvir = 1013.4-1014.0M⊙ 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 × 1011M⊙, which suggests that over half the stellar mass of its descendant had been formed by z = 3. Moreover, we have identified over two members for each of the four Lyα 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.
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