The Most Luminous z~9-10 Galaxy Candidates yet Found: The Luminosity
  Function, Cosmic Star-Formation Rate, and the First Mass Density Estimate at
  500 Myr
  [abridged] We present the discovery of four surprisingly bright (H_160 ~ 26 -
27 mag AB) galaxy candidates at z~9-10 in the complete HST CANDELS WFC3/IR
GOODS-N imaging data, doubling the number of z~10 galaxy candidates that are
known, just ~500 Myr after the Big Bang. Two similarly bright sources are also
detected in a systematic re-analysis of the GOODS-S data set. Three of the four
galaxies in GOODS-N are significantly detected at 4.5-6.2sigma in the very deep
Spitzer/IRAC 4.5 micron 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 micron (6.9sigma), revealing a flat UV spectral energy distribution with a
slope beta=-2.0+-0.2, consistent with demonstrated trends with luminosity at
high redshift. The abundance of such luminous candidates suggests that the
luminosity function evolves more significantly in phi_* than in L_* at z>~8
with a higher number density of bright sources than previously expected.
Despite the discovery of these luminous candidates, the cosmic star formation
rate density for galaxies with SFR >0.7 M_sun/yr shows an order-of-magnitude
increase in only 170 Myr from z ~ 10 to z ~ 8, consistent with previous
results. Based on the IRAC detections, we derive galaxy stellar masses at z~10,
finding that these luminous objects are typically 10^9 M_sun. The cosmic
stellar mass density at z~10 is log10 rho_* = 4.7^+0.5_-0.8 M_sun Mpc^-3 for
galaxies brighter than M_UV~-18. The remarkable brightness, and hence
luminosity, of these z~9-10 candidates highlights the opportunity for deep
spectroscopy to determine their redshift and nature, demonstrates the value of
additional search fields covering a wider area to understand star-formation in
the very early universe, and highlights the opportunities for JWST to map the
buildup of galaxies at redshifts much earlier than z~10.
