Faint U-Band Dropouts in the WFPC2 Parallels of the Hubble Ultra Deep Field

We combine data from the extremely deep Hubble Space Telescope U (F300W) image, obtained using WFPC2 as part of the parallel observations of the Hubble Ultra Deep Field campaign, with BVi images from the Great Observatories Origins Deep Survey to identify a sample of Lyman break galaxies in the redshift range 2.0 ≲ z ≲ 3.5. We use recent stellar population synthesis models with a wide variety of ages, metallicities, redshifts, and dust content, and a detailed representation of the H I cosmic opacity as a function of redshift to model the colors of galaxies in our combination of WFPC2 and ACS filters. Using these models, we derive improved color selection criteria that provide a clean selection of relatively unobscured star-forming galaxies in this redshift range. Our WFPC2 F300W image is the deepest image ever obtained at that wavelength. The 10 σ limiting magnitude measured over 0.2 arcsec2 is 27.5 mag in the WFPC2 F300W image, about 0.5 mag deeper than the F300W image in the Hubble Deep Field (HDF)-North. This extra depth relative to the HDFs allows us to directly probe the luminosity function about 0.5 mag deeper than the depth accessible with the HDF data along an independent line of sight. Our sample of star-forming galaxies with 2.0 ≲ z ≲ 3.5 includes 125 objects, the majority of which show clumpy morphologies. We measure a star formation rate density of 0.18 M⊙ yr-1 Mpc-3, marginally higher than the value measured for the HDFs.