  A substantial number of ultra-high redshift (8 < z < 17) galaxy candidates
have been detected with JWST, posing the question: are these observational
results surprising in the context of current galaxy formation models? We
address this question using the well-established Santa Cruz semi-analytic
models, implemented within merger trees from the new suite of cosmological
N-body simulations GUREFT, which were carefully designed for ultra-high
redshift studies. Using our fiducial models calibrated at z=0, we present
predictions for stellar mass functions, rest-frame UV luminosity functions, and
various scaling relations. We find that our (dust-free) models predict galaxy
number densities at z~11 (z~13) that are an order of magnitude (a factor of
~30) lower than the observational estimates. We estimate the uncertainty in the
observed number densities due to cosmic variance, and find that it leads to a
fractional error of ~20-30% at z=11 (~30-80% at z=14) for a 100 sq arcmin
field. We explore which processes in our models are most likely to be
rate-limiting for the formation of luminous galaxies at these early epochs,
considering the halo formation rate, gas cooling, star formation, and stellar
feedback, and conclude that it is mainly efficient stellar-driven winds. We
find that a modest boost of a factor of ~4 to the UV luminosities, which could
arise from a top-heavy stellar initial mass function, would bring our current
models into agreement with the observations. Adding a stochastic component to
the UV luminosity can also reconcile our results with the observations.
