Dark-ages reionization & galaxy formation simulation IV: UV luminosity
  functions of high-redshift galaxies
  In this paper we present calculations of the UV luminosity function from the
Dark-ages Reionization And Galaxy-formation Observables from Numerical
Simulations (DRAGONS) project, which combines N-body, semi-analytic and
semi-numerical modelling designed to study galaxy formation during the Epoch of
Reionization. Using galaxy formation physics including supernova feedback, the
model naturally reproduces the UV LFs for high-redshift star-forming galaxies
from $z{\sim}5$ through to $z{\sim}10$. We investigate the luminosity--star
formation rate (SFR) relation, finding that variable SFR histories of galaxies
result in a scatter around the median relation of $0.1$--$0.3$ dex depending on
UV luminosity. We find close agreement between the model and observationally
derived SFR functions. We use our calculated luminosities to investigate the
luminosity function below current detection limits, and the ionizing photon
budget for reionization. We predict that the slope of the UV LF remains steep
below current detection limits and becomes flat at
$M_\mathrm{UV}{\gtrsim}{-14}$. We find that $48$ ($17$) per cent of the total
UV flux at $z{\sim}6$ ($10$) has been detected above an observational limit of
$M_\mathrm{UV}{\sim}{-17}$, and that galaxies fainter than
$M_\mathrm{UV}{\sim}{-17}$ are the main source of ionizing photons for
reionization. We investigate the luminosity--stellar mass relation, and find a
correlation for galaxies with $M_\mathrm{UV}{<}{-14}$ that has the form
$M_*{\propto}10^{-0.47M_\mathrm{UV}}$, in good agreement with observations, but
which flattens for fainter galaxies. We determine the luminosity--halo mass
relation to be $M_\mathrm{vir}{\propto}10^{-0.35M_\mathrm{UV}}$, finding that
galaxies with $M_\mathrm{UV}{=}{-20}$ reside in host dark matter haloes of
$10^{11.0\pm 0.1}\mathrm{M_\odot}$ at $z{\sim}6$, and that this mass decreases
towards high redshift.
