Star Formation at Z ∼ 6 : the Udf - Parallel Acs Fields 1

We report on the i-dropouts detected in two exceptionally deep ACS fields (B435, V606, i775, and z850 with 10σ 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 down to z850,AB = 28.1, or 1.4 i-dropouts arcmin, with significant field-to-field variation (as expected from cosmic variance). This extends i-dropout searches some ∼0.9 further down the luminosity function than was possible in the GOODS field, netting a ∼7× 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 (z850,AB < 27.2) with scaled versions of the HDF-N + HDF-S U -dropouts. The best-fit is found with a (1+z) +0.50 −0.53 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 (∼ 1350Å) 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) in number and (1+ z) in luminosity, suggesting a rest-frame continuum UV luminosity density at z ∼ 6 which is just 0.38 −0.07× that at z ∼ 3.8. Our inclusion of size evolution makes the present estimate lower than previous z ∼ 6 estimates. Subject headings: galaxies: evolution — galaxies: high-redshift — galaxies: luminosity function, mass