The importance of galaxy formation histories in models of reionization
  Upcoming galaxy surveys and 21-cm experiments targeting high redshifts
$z\gtrsim 6$ are highly complementary probes of galaxy formation and
reionization. However, in order to expedite the large volume simulations
relevant for 21-cm observations, many models of galaxies within reionization
codes are entirely subgrid and/or rely on halo abundances only. In this work,
we explore the extent to which resolving and modeling individual galaxy
formation histories affects predictions both for the galaxy populations
detectable by upcoming surveys and the signatures of reionization accessible to
upcoming 21-cm experiments. We find that a common approach, in which galaxy
luminosity is assumed to be a function of halo mass only, is biased with
respect to models in which galaxy properties are evolved through time via
semi-analytic modeling and thus reflective of the diversity of assembly
histories that naturally arise in $N$-body simulations. The diversity of galaxy
formation histories also results in scenarios in which the brightest galaxies
do \textit{not} always reside in the centers of large ionized regions, as there
are often relatively low-mass halos undergoing dramatic, but short-term,
growth. This has clear implications for attempts to detect or validate the
21-cm background via cross correlation. Finally, we show that a hybrid approach
-- in which only halos hosting galaxies bright enough to be detected in surveys
are modeled in detail, with the rest modeled as an unresolved field of halos
with abundance related to large-scale overdensity -- is a viable way to
generate large-volume `simulations` well-suited to wide-area surveys and
current-generation 21-cm experiments targeting relatively large $k \lesssim 1 \
h \ \text{Mpc}^{-1}$ scales.
