  Current models of the formation of first galaxies predict low masses and
faint objects at extremely high redshifts, z=9-15. However, the first
observations of this epoch indicate a higher-than-expected number of bright
(sometimes massive) galaxies. Numerical simulations can help to elucidate the
mild evolution of the bright end of the UV luminosity function and they can
provide the link between the evolution of bright galaxies and variations of the
galaxy formation efficiency across different redshifts. We use the FirstLight
database of 377 zoom-in cosmological simulations of a mass-complete sample of
galaxies. Mock luminosities are estimated by a dust model constrained by
current observations of an evolution of the beta-MUV relation at high-z.
FirstLight contains a high number of bright galaxies, MUV<-20, consistent with
current data at z=6-13. The evolution of the UV cosmic density is driven by the
evolution of the galaxy efficiency and the relation between MUV and halo mass.
The efficiency of galaxy formation increases significantly with redshift at a
fixed halo mass because galactic halos at extremely high redshifts convert gas
into stars at a higher rate than at lower redshifts. The high gas densities in
galaxies at z>9 enable these high efficiencies. Our simulations predict higher
number densities of massive galaxies, Ms=10^9 Msun, than other models with
constant efficiency. Cosmological simulations of galaxy formation with
self-consistent models of star formation and feedback can reproduce the
different regimes of galaxy formation across cosmic history.
