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We use state-of-the-art simulations to explore the physical evolution ofgalaxies in the first billion years of cosmic time. First, we demonstrate thatour model reproduces the basic statistical properties of the observedLyman-break galaxy (LBG) population at z = 6 - 8, including the evolvingultra-violet (UV) luminosity function (LF), the stellar-mass density (SMD), andthe average specific star-formation rates (sSFR) of LBGs with M_{UV} < -18 (ABmag). Encouraged by this success we present predictions for the behaviour offainter LBGs extending down to M_{UV} <= -15 (as will be probed with the JamesWebb Space Telescope) and have interrogated our simulations to try to gaininsight into the physical drivers of the observed population evolution. We findthat mass growth due to star formation in the mass-dominant progenitor buildsup about 90% of the total z ~ 6 LBG stellar mass, dominating over the masscontributed by merging throughout this era. Our simulation suggests that theapparent "luminosity evolution" depends on the luminosity range probed: thesteady brightening of the bright end of the LF is driven primarily by genuinephysical luminosity evolution and arises due to a fairly steady increase in theUV luminosity (and hence star-formation rates) in the most massive LBGs.However, at fainter luminosities the situation is more complex, due in part tothe more stochastic star-formation histories of lower-mass objects; at thisend, the evolution of the UV LF involves a mix of positive and negativeluminosity evolution (as low-mass galaxies temporarily brighten then fade)coupled with both positive and negative density evolution (as new low-massgalaxies form, and other low-mass galaxies are consumed by merging). We alsopredict the average sSFR of LBGs should rise from sSFR = 4.5 Gyr^-1 at z = 6 toabout 11 Gyr^-1 by z = 9.

Simulating the assembly of galaxies at redshifts z = 6 - 12