A Candidate Brightest Proto-Cluster Galaxy at z = 3.03

We report the discovery of a very 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 brightness profile with angular separations of ~0.''8 (~25 h^-1 kpc comoving). In addition, high signal-to-noise ratio rest-frame UV spectroscopy shows evidence for ~5 components based on stellar photospheric and ISM absorption lines with a velocity dispersion of sigma ~460 km s^-1 for the three strongest components. Both the dynamics and high luminosity, as well as our analysis of a LCDM numerical simulation, suggest a very massive system with halo mass M ~ 10^13 M_solar. The simulation finds that all halos at z = 3 of this mass contain sub-halos in agreement with the properties of these observed components and that such systems typically evolve into M ~ 10^14 M_solar halos in groups and clusters by z = 0. This discovery provides a rare opportunity to study the properties and individual components of z ~ 3 systems that are likely to be the progenitors to brightest cluster galaxies.