  We conduct a detailed comparison of broad-band spectral energy distributions
of six z >= 5.5 galaxies against galaxies drawn from cosmological hydrodynamic
simulations. We employ a new tool called SPOC, which constrains the physical
properties of observed galaxies through a Bayesian likelihood comparison with
model galaxies. For five out of six observed z>=5.5 objects, our simulated
galaxies match the observations at least as well as simple star formation
histories such as tau-models, with similar favored values obtained for the
intrinsic physical parameters such as stellar mass and star formation rate, but
with substantially smaller uncertainties. Our results are broadly insensitive
to simulation choices for galactic outflows and dust reddening. Hence the
existence of early galaxies as observed is broadly consistent with current
hierarchical structure formation models. However, one of the six objects has
photometry that is best fit by a bursty SFH unlike anything produced in our
simulations, driven primarily by a high K-band flux. These findings illustrate
how SPOC provides a robust tool for optimally utilizing hydrodynamic
simulations (or any model that predicts galaxy SFHs) to constrain the physical
properties of individual galaxies having only photometric data, as well as
identify objects that challenge current models. (abridged)
