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 (B₄₃₅, V₆₀₆, i₇₇₅, and z₈₅₀ with 10 σ 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 arcmin² 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 ∼0.9 mag further down the luminosity function than was possible in the Great Observatories Origins Deep Survey (GOODS) fields, yielding a ∼7 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 +0.50 -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 luminosity function (LF) in the rest-frame continuum UV (∼1350 Å) 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 ∼ 3.8. Our inclusion of the size evolution makes the present estimate lower than previous z ∼ 6 estimates.