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

We report on the i-dropouts detected in two exceptionally deep ACS fields (B₄₃₅, V₆₀₆, i₇₇₅, and z₈₅₀ with 10 sigma limits of 28.8, 29.0, 28.5, and 27.8, respectively) taken in parallel with the UDF NICMOS observations. Using an i-z>1.4 cut, we find 30 i-dropouts over 21 arcmin^2 down to z_AB=28.1, or 1.4 i-dropouts arcmin⁻², with significant field-to-field variation (as expected from cosmic variance). This extends i-dropout searches some ~0.9^m further down the luminosity function than was possible in the GOODS field, netting a ~7x increase in surface density. An estimate of the size evolution for UV bright objects is obtained by comparing the composite radial flux profile of the bright i-dropouts (z<27.2) with scaled versions of the HDF-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 from both GOODS fields, we make incompleteness estimates and construct a z~6 LF in the rest-frame continuum UV (~1350 A) over a 3.5 magnitude baseline, finding a shape consistent with that found at lower redshift. To evaluate the evolution in the LF from z~3.8, we make comparisons against different scalings of a lower redshift B-dropout sample. Though a strong degeneracy is found between luminosity and density evolution, our best-fit model scales as (1+z)-2.8 in number and (1+z)^0.1 in luminosity, suggesting a rest-frame continuum UV luminosity density at z~6 which is just 0.38-0.07 +0.09x that at z~3.8. Our inclusion of size evolution makes the present estimate lower than previous z~6 estimates.