  We present a comprehensive analysis of z>8 galaxies based on ultra-deep
WFC3/IR data. We constrain the evolution of the UV luminosity function (LF) and
luminosity densities from z~11 to z~8 by exploiting all the WFC3/IR data over
the Hubble Ultra-Deep Field from the HUDF09 and the new HUDF12 program, in
addition to the HUDF09 parallel field data, as well as wider area WFC3/IR
imaging over GOODS-South. Galaxies are selected based on the Lyman Break
Technique in three samples centered around z~9, z~10 and z~11, with seven z~9
galaxy candidates, and one each at z~10 and z~11. We confirm a new z~10
candidate (with z=9.8+-0.6) that was not convincingly identified in our first
z~10 sample. The deeper data over the HUDF confirms all our previous z>~7.5
candidates as genuine high-redshift candidates, and extends our samples to
higher redshift and fainter limits (H_160~29.8 mag). We perform one of the
first estimates of the z~9 UV LF and improve our previous constraints at z~10.
Extrapolating the lower redshift UV LF evolution should have revealed 17 z~9
and 9 z~10 sources, i.e., a factor ~3x and 9x larger than observed. The
inferred star-formation rate density (SFRD) in galaxies above 0.7 M_sun/yr
decreases by 0.6+-0.2 dex from z~8 to z~9, in good agreement with previous
estimates. The low number of sources found at z>8 is consistent with a very
rapid build-up of galaxies across z~10 to z~8. From a combination of all
current measurements, we find a best estimate of a factor 10x decrease in the
SFRD from z~8 to z~10, following (1+z)^(-11.4+-3.1). Our measurements thus
confirm our previous finding of an accelerated evolution beyond z~8, and
signify a rapid build-up of galaxies with M_UV<-17.7 within only ~200 Myr from
z~10 to z~8, in the heart of cosmic reionization.
