Star formation at z∼6:: The Hubble Ultra Deep parallel fields

We report on the i-dropouts detected in two exceptionally deep Advanced Camera for Surveys fields (B-435,B- V-606, i(775), and z(850) with 10sigma limits of 28.8, 29.0, 28.5, and 27.8, respectively) taken in parallel with the Ultra Deep Field Near-Infrared Camera and Multi-Object Spectrometer observations. Using an i-z > 1.4 cut, we find 30 i-dropouts over 21 arcmin(2) down to z(850,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 similar to0.9 mag further down the luminosity function than was possible in the Great Observatories Origins Deep Survey ( GOODS) fields, yielding a similar to7 times 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(850,AB) < 27.2) with scaled versions of the Hubble Deep Field - North and - South 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 luminosity function (LF) in the rest-frame continuum UV (∼1350 &ANGS;) over a 3.5 mag 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. Although 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 that is just 0.38(-0.07)(+0.09) times that at z similar to 3.8. Our inclusion of the size evolution makes the present estimate lower than previous estimates.