Abstract

We present the discovery of four surprisingly bright (H-160 similar to 26-27 mag AB) galaxy candidates at z similar to 9-10 in the complete HST CANDELS WFC3/IR GOODS-N imaging data, doubling the number of z similar to 10 galaxy candidates that are known, just similar to 500 Myr after the big bang. Two similarly bright sources are also detected in a reanalysis of the GOODS-S data set. Three of the four galaxies in GOODS-N are significantly detected at 4.5 sigma-6.2 sigma in the very deep Spitzer/IRAC 4.5 mu m data, as is one of the GOODS-S candidates. Furthermore, the brightest of our candidates (at z = 10.2 +/- 0.4) is robustly detected also at 3.6 mu m (6.9 sigma), revealing a flat UV spectral energy distribution with a slope beta = -2.0 +/- 0.2, consistent with demonstrated trends with luminosity at high redshift. Thorough testing and use of grism data excludes known low-redshift contamination at high significance, including single emission-line sources, but as-yet unknown low redshift sources could provide an alternative solution given the surprising luminosity of these candidates. Finding such bright galaxies at z similar to 9-10 suggests that the luminosity function for luminous galaxies might evolve in a complex way at z > 8. The cosmic star formation rate density still shows, however, an order-of-magnitude increase from z similar to 10 to z similar to 8 since the dominant contribution comes from low-luminosity sources. Based on the IRAC detections, we derive galaxy stellar masses at z similar to 10, finding that these luminous objects are typically 10(9) M-circle dot. This allows for a first estimate of the cosmic stellar mass density at z similar to 10 resulting in log(10) P* = 4.7(-0.8)(+0.5) M-circle dot Mpc(-3) for galaxies brighter than MUV similar to -18. The remarkable brightness, and hence luminosity, of these z similar to 9-10 candidates will enable deep spectroscopy to determine their redshift and nature, and highlights the opportunity for the James Webb Space Telescope to map the buildup of galaxies at redshifts much earlier than z similar to 10.