  We present estimates of the ultraviolet (UV) and Lyman continuum flux density
contributed by galaxies of luminosities from $M_{\rm UV}\approx -25$ to $M_{\rm
UV}=-4$ at redshifts $5\leq z\leq 10$ using a galaxy formation model that
reproduces properties of local dwarf galaxies down to the luminosities of the
ultra-faint satellites. We characterize the UV luminosity function (LF) of
galaxies and their abundance as a function of the ionizing photon emission rate
predicted by our model and present accurate fitting functions describing them.
Although the slope of the LF becomes gradually shallower with decreasing
luminosity due to feedback-driven outflows, the UV LF predicted by the model
remains quite steep at the luminosities $M_{\rm UV}\lesssim -14$. After
reionization, the UV LF flattens at $M_{\rm UV}\gtrsim -12$ due to UV heating
of intergalactic gas. However, before reionization, the slope of the LF remains
steep and approximately constant from $M_{\rm UV}\approx -14$ to $M_{\rm
UV}=-4$. We show that for a constant ionizing photon escape fraction the
contribution of faint galaxies with $M_{\rm UV}>-14$ to the UV flux and
ionizing photon budget is $\approx 40-60\%$ at $z>7$ and decreases to $\approx
20\%$ at $z=6$. Before reionization, even ultra-faint galaxies of $M_{\rm
UV}>-10$ contribute $\approx 10-25\%$ of ionizing photons. If the escape
fraction increases strongly for fainter galaxies, the contribution of $M_{\rm
UV}>-14$ galaxies before reionization increases to $\approx 60-75\%$. Our
results imply that dwarf galaxies fainter than $M_{\rm UV}=-14$, beyond the
James Webb Space Telescope limit, contribute significantly to the UV flux
density and ionizing photon budget before reionization alleviating requirements
on the escape fraction of Lyman continuum photons.
