The z = 9-10 galaxy population in the Hubble Frontier Fields and CLASH
  surveys: The z=9 LF and further evidence for a smooth decline in UV
  luminosity at z >= 8
  We present the results of a search for z=9-10 galaxies within the first 8
pointings of the Hubble Frontier Fields (HFF) (4 clusters plus 4 parallel
fields) and 20 cluster fields from the CLASH survey. Combined with our previous
analysis of the Hubble Ultra-Deep field (HUDF), we have now completed a search
for z=9-10 galaxies over ~130 sq. arcmin, across 29 HST WFC3/IR pointings. As
in our recent study of the first two HFF fields, we confine our primary search
for high-redshift candidates in the HFF imaging to the uniformly deep (i.e.
sigma_160>30 AB mag in 0.5-arcsec diameter apertures), relatively low
magnification regions. In the CLASH fields our search was confined to uniformly
deep regions where sigma_160>28.8 AB mag. Our SED fitting analysis unveils a
sample of 33 galaxy candidates at z_phot>=8.4, five of which have primary
photometric redshift solutions in the range 9.6<z_phot<11.2. By calculating a
de-lensed effective volume for each candidate, the improved statistics and
reduced cosmic variance provided by our new sample allows a more accurate
determination of the UV-selected galaxy luminosity function (LF) at z~9. Our
new results strengthen our previous conclusion that the LF appears to evolve
smoothly from z=8 to z=9, an evolution which can be equally well modelled by a
factor of ~2 drop in density, or a dimming of ~0.5 mag in M*. Moreover, based
on our new sample, we are able to place initial constraints on the z=10 LF,
finding that the number density at M_1500 ~ -19.7 is log(phi) = -4.1
(+0.2,-0.3), a factor of ~2 lower than at z=9. Finally, we use our new results
to re-visit the issue of the decline in UV luminosity density at z>=8. We
conclude that the data continue to support a smooth decline in rho_UV over the
redshift interval 6<z<10, in agreement with simple models of early galaxy
evolution driven by the growth in the underlying dark matter halo mass
function.
