A Candidate Brightest Protocluster Galaxy at z = 3.03

We report the discovery of a bright (m_R = 22.2) Lyman break galaxy at z = 3.03 that appears to be a massive system in a late stage of merging. Deep imaging reveals multiple peaks in the flux profile with angular separations of ~0.8^'' (~20 h⁻¹ kpc, comoving). In addition, high signal-to-noise ratio rest-frame UV spectroscopy shows evidence for at least three individual components based on stellar photospheric and ISM absorption lines. We find a 1D velocity dispersion of σ ∼ 450 km s⁻¹ for the three strongest components. Both the dynamics and high luminosity as well as our analysis of a ΛCDM numerical simulation suggest that this is a system with halo mass M ∼ 10¹³ M_☉. We find in the simulation that all halos of this mass at z = 3 contain massive subhalos that agree with the observed component properties. These halos typically evolve into M ∼ 10¹⁴–10^14.5 M_☉ halos in groups and clusters by z = 0. This discovery provides a rare opportunity to study the properties and components of a z ∼ 3 system that is likely to be the progenitor of a brightest cluster galaxy.