  We present the results of a numerical study comparing photometric and
physical properties of simulated z=6-9 galaxies to the observations taken by
the WFC3 instrument aboard the Hubble Space Telescope. Using cosmological
hydrodynamical simulations we find good agreement with observations in
color-color space at all studied redshifts. We also find good agreement between
observations and our Schechter luminosity function fit in the observable range,
Muv<= -18, provided that a moderate dust extinction effect exists for massive
galaxies. However beyond what currently can be observed, simulations predict a
very large number of low-mass galaxies and evolving steep faint-end slopes from
alpha_L = -2.15 at z=6 to alpha_L = -2.64 at z=9, with a dependence of
|alpha_L| \propto (1+z)^0.59. During the same epoch, the normalization phi*
increases and the characteristic magnitude Muv* becomes moderately brighter
with decreasing redshift. We find similar trends for galaxy stellar mass
function with evolving low-mass end slope from alpha_M = - 2.26 at z=6 to
alpha_M = -2.87 at z=9, with a dependence of |alpha_M| \propto (1+z)^0.65.
Together with our recent result on the high escape fraction of ionizing photons
for low-mass galaxies, our results suggest that the low-mass galaxies are
important contributor of ionizing photons for the reionisation of the Universe
at z>=6.
