Abstract

We present new z similar to 8 galaxy candidates from a search over similar to 95 arcmin(2) of WFC3/IR data, tripling the previous search area for bright z similar to 8 galaxies. Our analysis uses newly acquiredWFC3/IR imaging data from the CANDELS Multi-CycleTreasury program over the GOODS-South field. These new data are combined with existing deep optical Advanced Camera for Surveys (ACS) imaging to search for relatively bright (M-UV -19.5 mag) z similar to 8 galaxy candidates using the Lyman break technique. These new candidates are used to determine the bright end of the UV luminosity function (LF) of star-forming galaxies at z similar to 7.2-8.7, i.e., a cosmic age of 600 +/- 80 Myr. To minimize contamination from lower redshift galaxies, we make full use of all optical ACS data and impose strict non-detection criteria based on an optical chi(2)(opt) flux measurement. In the whole search area, we identify 16 candidate z similar to 8 galaxies, spanning a magnitude range H-160,(AB) = 25.7-27.9 mag. The new data show that the UV LF is a factor similar to 1.7 lower at M-UV -19.5 mag than determined from the HUDF09 and Early Release Science (ERS) data alone. Combining this new sample with the previous candidates from the HUDF09 and ERS data allows us to perform the most accurate measurement of the z similar to 8 UV LF yet. Schechter function fits to the combined data result in a best-fit characteristic magnitude of M-*(z = 8) = similar to 20.04 +/- 0.46 mag. The faint-end slope is very steep, though quite uncertain, with alpha = -2.06 +/- 0.32. A combination of wide-area data with additional ultra-deep imaging will be required to significantly reduce the uncertainties on these parameters in the future.