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We report the discovery of 24 Lyman-break candidates at 7<z<10.5, in theHubble Frontier Fields (HFF) imaging data of Abell 2744 (z=0.308), plusSpiter/IRAC data and archival ACS data. The sample includes a triple imagesystem with a photometric redshift of z~7.4. This high redshift isgeometrically confirmed by our lens model corresponding to deflection anglesthat are 12% larger than the lower-redshift systems used to calibrate the lensmodel at z=2.019. The majority of our high-redshift candidates are not expectedto be multiply lensed given their locations in the image plane and thebrightness of foreground galaxies, but are magnified by factors of ~1.3-15, sothat we are seeing further down the luminosity function than comparable deepfield imaging. It is apparent that the redshift distribution of these sourcesdoes not smoothly extend over the full redshift range accessible at z<12, butappears to break above z=9. Nine candidates are clustered within a small regionof 20" across, representing a potentially unprecedented concentration. Giventhe poor statistics, however, we must await similar constraints from theadditional HFF clusters to properly examine this trend. The physical propertiesof our candidates are examined using the range of lens models developed for theHFF program by various groups including our own, for a better estimate ofunderlying systematics. Our spectral-energy-distribution fits for the brightestobjects suggest stellar masses of ~1E9 solar mass, star-formation rates of ~4solar mass per yr, and a typical formation redshift of z<19.

Young Galaxy Candidates in the Hubble Frontier Fields. I. Abell 2744