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 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 ~ 1014–1014.5 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.
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