Understanding the Observed Evolution of the Galaxy Luminosity Function
  from z=6-10 in the Context of Hierarchical Structure Formation
  Recent observations of the Lyman-break galaxy (LBG) luminosity function (LF)
from z~6-10 show a steep decline in abundance with increasing redshift.
However, the LF is a convolution of the mass function of dark matter halos
(HMF)--which also declines sharply over this redshift range--and the
galaxy-formation physics that maps halo mass to galaxy luminosity. We consider
the strong observed evolution in the LF from z~6-10 in this context and
determine whether it can be explained solely by the behavior of the HMF. From
z~6-8, we find a residual change in the physics of galaxy formation
corresponding to a ~0.5 dex increase in the average luminosity of a halo of
fixed mass. On the other hand, our analysis of recent LF measurements at z~10
shows that the paucity of detected galaxies is consistent with almost no change
in the average luminosity at fixed halo mass from z~8. The LF slope also
constrains the variation about this mean such that the luminosity of galaxies
hosted by halos of the same mass are all within about an order-of-magnitude of
each other. We show that these results are well-described by a simple model of
galaxy formation in which cold-flow accretion is balanced by star formation and
momentum-driven outflows. If galaxy formation proceeds in halos with masses
down to 10^8 Msun, then such a model predicts that LBGs at z~10 should be able
to maintain an ionized intergalactic medium as long as the ratio of the
clumping factor to the ionizing escape fraction is C/f_esc < 10.
