A Candidate Brightest Protocluster Galaxy at z = 3.03

Abstract

We report the discovery of a bright (mR = 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 sigma~450 km s-1 for the three strongest components. Both the dynamics and high luminosity as well as our analysis of a LambdaCDM numerical simulation suggest that this is a system with halo mass M~1013 Msolar. 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~1014-1014.5 Msolar 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.