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.
