  The Hubble Frontier Fields (HFF) program combines the capabilities of the
Hubble Space Telescope (HST) with the gravitational lensing of massive galaxy
clusters to probe the distant Universe to an unprecedented depth. Here, we
present the results of the first combined HST and Spitzer observations of the
cluster Abell 2744. We combine the full near-infrared data with ancillary
optical images to search for gravitationally lensed high-redshift (z > 6)
galaxies. We report the detection of 15 I814-dropout candidates at z ~ 6-7 and
one Y105-dropout at z ~ 8 in a total survey area of 1.43 arcmin^2 in the source
plane. The predictions of our lens model allow us to also identify five
multiply-imaged systems lying at redshifts between z ~ 6 and z ~ 8. Thanks to
constraints from the mass distribution in the cluster, we were able to estimate
the effective survey volume corrected for completeness and magnification
effects. This was in turn used to estimate the rest-frame ultraviolet
luminosity function (LF) at z ~ 6-8. Our LF results are generally in agreement
with the most recent blank field estimates, confirming the feasibility of
surveys through lensing clusters. Although based on a shallower observations
than what will be achieved in the final dataset including the full ACS
observations, the LF presented here extends down to Muv ~ -18.5 at z ~ 7 with
one identified object at Muv ~ -15 thanks to the highly-magnified survey areas.
This early study forecasts the power of using massive galaxy clusters as cosmic
telescopes and its complementarity to blank fields.
