Evolution in the Colors of Lyman-Break Galaxies from Z~4 to Z~3

The integrated colors of distant galaxies provide a means for interpreting the properties of their stellar content. Here, we use rest-frame UV-to-optical colors to constrain the spectral-energy distributions and stellar populations of color-selected, B-dropout galaxies at z ~ 4 in the Great Observatories Origins Deep Survey. We combine the ACS data with ground-based near-infrared images, which extend the coverage of galaxies at z ~ 4 to the rest-frame B-band. We observe a color-magnitude trend in the rest-frame m(UV) - B versus B diagram for the z ~ 4 galaxies that has a fairly well-defined "blue-envelope", and is strikingly similar to that of color-selected, U-dropout galaxies at z ~ 3. We also find that although the co-moving luminosity density at rest-frame UV wavelengths (1600 Angstroms) is roughly comparable at z ~ 3 and z ~ 4, the luminosity density at rest-frame optical wavelengths increases by about one-third from z ~ 4 to z ~ 3. Although the star-formation histories of individual galaxies may involve complex and stochastic events, the evolution in the global luminosity density of the UV-bright galaxy population corresponds to an average star-formation history with a star-formation rate that is constant or increasing over these redshifts. This suggests that the evolution in the luminosity density corresponds to an increase in the stellar-mass density of more than 33%.