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

We report the discovery of 24 Lyman-break candidates at 7<z<10.5, in the Hubble Frontier Fields (HFF) imaging data of Abell 2744 (z=0.308), plus Spiter/IRAC data and archival ACS data. The sample includes a triple image system with a photometric redshift of z~7.4. This high redshift is geometrically confirmed by our lens model corresponding to deflection angles that are 12% larger than the lower-redshift systems used to calibrate the lens model at z=2.019. The majority of our high-redshift candidates are not expected to be multiply lensed given their locations in the image plane and the brightness of foreground galaxies, but are magnified by factors of ~1.3-15, so that we are seeing further down the luminosity function than comparable deep field imaging. It is apparent that the redshift distribution of these sources does not smoothly extend over the full redshift range accessible at z<12, but appears to break above z=9. Nine candidates are clustered within a small region of 20" across, representing a potentially unprecedented concentration. Given the poor statistics, however, we must await similar constraints from the additional HFF clusters to properly examine this trend. The physical properties of our candidates are examined using the range of lens models developed for the HFF program by various groups including our own, for a better estimate of underlying systematics. Our spectral-energy-distribution fits for the brightest objects suggest stellar masses of ~1E9 solar mass, star-formation rates of ~4 solar mass per yr, and a typical formation redshift of z<19.