New redshift z ~ 9 galaxies in the Hubble Frontier Fields: Implications
  for early evolution of the UV luminosity density
  We present the results of a new search for galaxies at redshift z ~ 9 in the
first two Hubble Frontier Fields with completed HST WFC3/IR and ACS imaging. To
ensure robust photometric redshift solutions, and to minimize incompleteness,
we confine our search to objects with H_{160} < 28.6 (AB mag), consider only
image regions with an rms noise sigma_{160} > 30 mag (within a 0.5-arcsec
diameter aperture), and insist on detections in both H_{160} and J_{140}. The
result is a survey covering an effective area (after accounting for
magnification) of 10.9 sq. arcmin, which yields 12 galaxies at 8.4 < z < 9.5.
Within the Abell-2744 cluster and parallel fields we confirm the three
brightest objects reported by Ishigaki et al. (2014), but recover only one of
the four z > 8.4 sources reported by Zheng et al. (2014). In the
MACSJ0416.1-240 cluster field we report five objects, and explain why each of
these eluded detection or classification as z ~ 9 galaxies in the published
searches of the shallower CLASH data. Finally, we uncover four z ~ 9 galaxies
from the previously unsearched MACSJ0416.1-240 parallel field. Based on the
published magnification maps we find that only one of these 12 galaxies is
likely boosted by more than a factor of two by gravitational lensing.
Consequently we are able to perform a fairly straightforward reanalysis of the
normalization of the z ~ 9 UV galaxy luminosity function as explored previously
in the HUDF12 programme. We conclude that the new data strengthen the evidence
for a continued smooth decline in UV luminosity density (and hence
star-formation rate density) from z ~ 8 to z ~ 9, contrary to recent reports of
a marked drop-off at these redshifts. This provides further support for the
scenario in which early galaxy evolution is sufficiently extended to explain
cosmic reionization.
