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We report on the i-dropouts detected in two exceptionally deep ACS fields(B_{435}, V_{606}, i_{775}, and z_{850} with 10 sigma limits of 28.8, 29.0,28.5, and 27.8, respectively) taken in parallel with the UDF NICMOSobservations. Using an i-z>1.4 cut, we find 30 i-dropouts over 21 arcmin^2 downto z_AB=28.1, or 1.4 i-dropouts arcmin^{-2}, with significant field-to-fieldvariation (as expected from cosmic variance). This extends i-dropout searchessome ~0.9^m further down the luminosity function than was possible in the GOODSfield, netting a ~7x increase in surface density. An estimate of the sizeevolution for UV bright objects is obtained by comparing the composite radialflux profile of the bright i-dropouts (z<27.2) with scaled versions of theHDF-N + HDF-S U-dropouts. The best-fit is found with a (1+z)^{-1.57_{-0.53}^{+0.50}} scaling in size (for fixed luminosity), extending lower redshift(1<z<5) trends to z~6. Adopting this scaling and the brighter i-dropouts fromboth GOODS fields, we make incompleteness estimates and construct a z~6 LF inthe rest-frame continuum UV (~1350 A) over a 3.5 magnitude baseline, finding ashape consistent with that found at lower redshift. To evaluate the evolutionin the LF from z~3.8, we make comparisons against different scalings of a lowerredshift B-dropout sample. Though a strong degeneracy is found betweenluminosity and density evolution, our best-fit model scales as (1+z)^{-2.8} innumber and (1+z)^0.1 in luminosity, suggesting a rest-frame continuum UVluminosity density at z~6 which is just 0.38_{-0.07} ^{+0.09}x that at z~3.8.Our inclusion of size evolution makes the present estimate lower than previousz~6 estimates.Comment: 5 pages, 5 figures, accepted for publication in the Astrophysical  Journal Letters, labelling to the left-hand axis of Figure 4 correcte

Star Formation at z ~ 6: The Hubble Ultra Deep Parallel Fields