Probing $\sim L_{*}$ Lyman-break Galaxies at $z\approx 7$ in GOODS-South
  with WFC3 on HST
  We analyse recently acquired near-infrared Hubble Space Telescope imaging of
the GOODS-South field to search for star forming galaxies at z~7.0. By
comparing WFC 3 0.98 micron Y-band images with ACS z-band (0.85 micron) images,
we identify objects with colours consistent with Lyman break galaxies at
z~6.4-7.4. This new data covers 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 WFC3 J-band we attempt to remove any low-redshift interlopers. Our
selection criteria yields 6 candidates brighter than Y_AB = 27.0, of which all
except one are detected in the ACS z-band imaging and are thus unlikely to be
transients. Assuming all 6 candidates are at z~7 this implies a surface density
of objects brighter than Y_AB = 27.0 of 0.30\pm0.12 arcmin-2, a value
significantly smaller than the prediction from the z ~ 6 luminosity function.
This suggests continued evolution of the bright end of the luminosity function
between z = 6 to 7, with number densities lower at higher redshift.
