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 similar to 0.8 '' (similar to 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 sigma similar to 450 km s(-1) for the three strongest components. Both the dynamics and high luminosity as well as our analysis of a Lambda CDM numerical simulation suggest that this is a system with halo mass. M similar to 10(13) M-circle dot. We find in the simulation that all halos of this mass at contain massive subhalos that agree with the observed component properties. These halos typically evolve into M similar to 10(14)-10(14.5) M-circle dot halos in groups and clusters by z = 0. This discovery provides a rare opportunity to study the properties and components of a z similar to 3 system that is likely to be the progenitor of a brightest cluster galaxy.