  Using an i-z dropout criterion, we determine the space density of z~6
galaxies from two deep ACS GTO fields with deep optical-IR imaging. A total of
23 objects are found over 46 arcmin^2, or ~0.5 objects/arcmin^2 down to z~27.3
(6 sigma; all AB mag) (including one probable z~6 AGN). Combining deep ISAAC
data for our RDCS1252-2927 field (J~25.7 and Ks~25.0 (5 sigma)) and NICMOS data
for the HDF North (JH~27.3 (5 sigma)), we verify that these dropouts have flat
spectral slopes. i-dropouts in our sample range in luminosity from ~1.5 L*
(z~25.6) to ~0.3 L* (z~27.3) with the exception of one very bright candidate at
z~24.2. The half-light radii vary from 0.09" to 0.29", or 0.5 kpc to 1.7 kpc.
We derive the z~6 rest-frame UV luminosity density using three different
procedures, each utilizing simulations based on a CDF South V dropout sample.
First, we compare our findings with a no-evolution projection of this V-dropout
sample. We find 23+/-25% more i-dropouts than we predict. Adopting previous
results to z~5, this works out to a 20+/-29% drop in the luminosity density
from z~3 to z~6. Second, we use these same V-dropout simulations to derive a
selection function for our i-dropout sample and compute the UV-luminosity
density (7.2+/-2.5 x 10^25 ergs/s/Hz/Mpc^3 down to z~27). We find a 39+/-21%
drop over the same redshift range. This is our preferred value and suggests a
star formation rate of 0.0090+/-0.0031 M_sol/yr/Mpc^3 to z~27, or ~0.036+/-
0.012 M_sol/yr/Mpc^3 extrapolating the LF to the faint limit. Third, we follow
a very similar procedure, but assume no incompleteness, finding a luminosity
density which is ~2-3X lower. This final estimate constitutes a lower limit.
All three estimates are within the canonical range of luminosity densities
necessary for reionization of the universe at this epoch. (abridged)
