Simulating the assembly of galaxies at redshifts z = 6 - 12
  We use state-of-the-art simulations to explore the physical evolution of
galaxies in the first billion years of cosmic time. First, we demonstrate that
our model reproduces the basic statistical properties of the observed
Lyman-break galaxy (LBG) population at z = 6 - 8, including the evolving
ultra-violet (UV) luminosity function (LF), the stellar-mass density (SMD), and
the average specific star-formation rates (sSFR) of LBGs with M_{UV} < -18 (AB
mag). Encouraged by this success we present predictions for the behaviour of
fainter LBGs extending down to M_{UV} <= -15 (as will be probed with the James
Webb Space Telescope) and have interrogated our simulations to try to gain
insight into the physical drivers of the observed population evolution. We find
that mass growth due to star formation in the mass-dominant progenitor builds
up about 90% of the total z ~ 6 LBG stellar mass, dominating over the mass
contributed by merging throughout this era. Our simulation suggests that the
apparent "luminosity evolution" depends on the luminosity range probed: the
steady brightening of the bright end of the LF is driven primarily by genuine
physical luminosity evolution and arises due to a fairly steady increase in the
UV luminosity (and hence star-formation rates) in the most massive LBGs.
However, at fainter luminosities the situation is more complex, due in part to
the more stochastic star-formation histories of lower-mass objects; at this
end, the evolution of the UV LF involves a mix of positive and negative
luminosity evolution (as low-mass galaxies temporarily brighten then fade)
coupled with both positive and negative density evolution (as new low-mass
galaxies form, and other low-mass galaxies are consumed by merging). We also
predict the average sSFR of LBGs should rise from sSFR = 4.5 Gyr^-1 at z = 6 to
about 11 Gyr^-1 by z = 9.
