UV-DROPOUT GALAXIES IN THE GOODS-SOUTH FIELD FROM WFC3 EARLY RELEASE SCIENCE OBSERVATIONS

We combine new high sensitivity ultraviolet (UV) imaging from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with existing deep HST/Advanced Camera for Surveys optical images from the Great Observatories Origins Deep Survey (GOODS) program to identify UV-dropouts, which are Lyman break galaxy (LBG) candidates at z similar or equal to 1-3. These new HST/WFC3 observations were taken over 50 arcmin(2) in the GOODS-South field as a part of the Early Release Science program. The uniqueness of these new UV data is that they are observed in three UV/optical (WFC3 UVIS) channel filters (F225W, F275W, and F336W), which allows us to identify three different sets of UV-dropout samples. We apply Lyman break dropout selection criteria to identify F225W-, F275W-, and F336W-dropouts, which are z similar or equal to 1.7, 2.1, and 2.7 LBG candidates, respectively. We use multi-wavelength imaging combined with available spectroscopic and photometric redshifts to carefully access the validity of our UV-dropout candidates. Our results are as follows: (1) these WFC3 UVIS filters are very reliable in selecting LBGs with z similar or equal to 2.0, which helps to reduce the gap between the well-studied z greater than or similar to 3 and z similar to 0 regimes; (2) the combined number counts with average redshift z similar or equal to 2.2 agree very well with the observed change in the surface densities as a function of redshift when compared with the higher redshift LBG samples; and (3) the best-fit Schechter function parameters from the rest-frame UV luminosity functions at three different redshifts fit very well with the evolutionary trend of the characteristic absolute magnitude, M*, and the faint-end slope, alpha, as a function of redshift. This is the first study to illustrate the usefulness of the WFC3 UVIS channel observations to select z less than or similar to 3 LBGs. The addition of the new WFC3 on the HST has made it possible to uniformly select LBGs from z similar or equal to 1 to z similar or equal to 9 and significantly enhance our understanding of these galaxies using HST sensitivity and resolution.