FirstLight III: Rest-frame UV-optical spectral energy distributions of
  simulated galaxies at cosmic dawn
  Using the FirstLight database of 300 zoom-in cosmological simulations we
provide rest-frame UV-optical spectral energy distributions of galaxies with
complex star-formation histories that are coupled to the non-uniform gas
accretion history of galactic halos during cosmic dawn. The population at any
redshift is very diverse ranging from starbursts to quiescent galaxies even at
a fixed stellar mass. This drives a redshift-dependent relation between UV
luminosity and stellar mass with a large scatter, driven by the specific star
formation rate. The UV slope and the production efficiency of Lyman continuum
photons have high values, consistent with dust-corrected observations. This
indicates young stellar populations with low metallicities. The FirstLight
simulations make predictions on the rest-frame UV-optical absolute magnitudes,
colors and optical emission lines of galaxies at z=6-12 that will be observed
for the first time with JWST and the next generation of telescopes in the
coming decade.
