Ultraviolet Luminosity Density of the Universe During the Epoch of
  Reionization
  The spatial fluctuations of the extragalactic background light trace the
total emission from all stars and galaxies in the Universe. A multi-wavelength
study can be used to measure the integrated emission from first galaxies during
reionization when the Universe was about 500 million years old. Here we report
arcminute-scale spatial fluctuations in one of the deepest sky surveys with the
Hubble Space Telescope in five wavebands between 0.6 and 1.6 $\mu$m. We
model-fit the angular power spectra of intensity fluctuation measurements to
find the ultraviolet luminosity density of galaxies at $z$ > 8 to be $\log
\rho_{\rm UV} = 27.4^{+0.2}_{-1.2}$ erg s$^{-1}$ Hz$^{-1}$ Mpc$^{-3}$
$(1\sigma)$. This level of integrated light emission allows for a significant
surface density of fainter primeval galaxies that are below the point source
detection level in current surveys.
