Probing ~L* Lyman-break galaxies at z˜ 7 in GOODS-South with WFC3 on Hubble Space Telescope

We analyse recently acquired near-infrared Hubble Space Telescope imaging of the Great Observatories Origins Deep Survey (GOODS)-South field to search for star-forming galaxies at z˜ 7.0. By comparing Wide Field Camera 3 (WFC3) 0.98 µm Y-band images with Advanced Camera for Surveys (ACS) z-band (0.85 µm) images, we identify objects with colours consistent with Lyman-break galaxies at z? 6.4–7.4. This new data cover an area five times larger than that previously reported in the WFC3 imaging of the Hubble Ultra Deep Field and affords a valuable constraint on the bright end of the luminosity function. Using additional imaging of the region in the ACS B, V and i bands from GOODS v2.0 and the WFC3J band, we attempt to remove any low-redshift interlopers. Our selection criteria yields six candidates brighter than YAB= 27.0, of which all except one are detected in the ACS z-band imaging and are thus unlikely to be transients. Assuming all six candidates are at z˜ 7, this implies a surface density of objects brighter than YAB= 27.0 of 0.30 ± 0.12 arcmin-2, a value significantly smaller than the prediction from z˜ 6 luminosity function. This suggests continued evolution of the bright end of the luminosity function between z= 6 and 7, with number densities lower at higher redshift.