  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.
