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

Abstract

We report on the i-dropouts detected in two exceptionally deep Advanced Camera for Surveys 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 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 z850,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 (z850,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