Smoothly-Rising Star Formation Histories During the Reionization Epoch
  Cosmological hydrodynamic simulations robustly predict that high-redshift
galaxy star formation histories (SFHs) are smoothly-rising and vary with mass
only by a scale factor. We use our latest simulations to test whether this
scenario can account for recent observations at z>=6 from WFC3/IR, NICMOS, and
IRAC. Our simulations broadly reproduce the observed ultraviolet (UV)
luminosity functions and stellar mass densities and their evolution at z=6-8,
all of which are nontrivial tests of the mean SFH. In agreement with
observations, simulated galaxies possess blue UV continua owing to young ages
(50-150 Myr), low metallicities (0.1-0.5 Zsun), and low dust columns (E(B-V) <=
0.05). Observations imply a near-unity slope in the stellar mass--star
formation rate relation at all z=6-8, confirming the prediction that SFH shapes
are invariant. Current surveys detect the majority of galaxies with stellar
masses exceeding 10^9 Msun and few galaxies less massive than 10^{8.5} Msun,
implying that they probe no more than the brightest 30% of the complete star
formation and stellar mass densities at z>=6. Finally, we demonstrate that
there is no conflict between smoothly-rising SFHs and recent clustering
observations. This is because momentum-driven outflows suppress star formation
in low-mass halos, leading to overall occupancies of 0.2-0.4 even though the
star formation duty cycle is one. This leads to many interesting predictions at
z>=4, among them that (1) optically-selected and UV-selected samples largely
overlap; (2) few galaxies exhibit significantly suppressed specific star
formation rates; and (3) occupancy is constant or increasing with decreasing
luminosity. These predictions are in tentative agreement with current
observations, but further analysis of existing and upcoming data sets is
required in order to test them more thoroughly. (abridged)
