The Bright End of the Ultraviolet Luminosity Function at z ~ 8: New Constraints from CANDELS Data in GOODS-South

We present new z ~ 8 galaxy candidates from a search over ~95 arcmin² of WFC3/IR data, tripling the previous search area for bright z ~ 8 galaxies. Our analysis uses newly acquired WFC3/IR imaging data from the CANDELS Multi-Cycle Treasury 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 ~ 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 ~ 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 χ² _opt flux measurement. In the whole search area, we identify 16 candidate z ~ 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 ~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 ~ 8 UV LF yet. Schechter function fits to the combined data result in a best-fit characteristic magnitude of M _*(z = 8) = -20.04 ± 0.46 mag. The faint-end slope is very steep, though quite uncertain, with α = -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.

Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555.