AstraeusIV : Quantifying the star formation histories of galaxies in the
  Epoch of Reionization
  We use the \textsc{astraeus} framework, that couples an N-body simulation
with a semi-analytic model for galaxy formation and a semi-numerical model for
reionization, to quantify the star formation histories (SFHs) of galaxies in
the first billion years. Exploring four models of radiative feedback, we fit
the SFH of each galaxy at $z>5$ as $\mathrm{log}(\mathrm{SFR}(z))=-\alpha(1 +
z)+\beta$; star formation is deemed stochastic if it deviates from this fit by
more than $\Delta_\mathrm{SFR}=0.6\,$dex. Our key findings are: (i) The
fraction of stellar mass formed and time spent in the stochastic phase decrease
with increasing stellar mass and redshift $z$. While galaxies with stellar
masses of $M_\star\sim10^7M_\odot$ at $z\sim5~(10)$ form $\sim70\%~(20\%)$ of
their stellar mass in the stochastic phase, this reduces to $<10\%$ at all
redshifts for galaxies with $M_\star > 10^{10}M_\odot$; (ii) the fractional
mass assembled and lifetime spent in the stochastic phase do not significantly
change with the radiative feedback model used; (iii) at all redshifts, $\alpha$
increases (decreases for the strongest radiative feedback model) with stellar
mass for galaxies with $M_\star\lesssim 10^{8.5}M_\odot$ and converges to
$\sim0.18$ for more massive galaxies; $\beta$ always increases with stellar
mass. Our proposed fits can reliably recover the stellar masses and
mass-to-light ratios for galaxies with $M_\star\sim10^{8-10.5}M_\odot$ and
$M_{UV}\sim-17~{\rm to}~-23$ at $z\sim 5-9$. This physical model can therefore
be used to derive the SFHs for galaxies observed by a number of forthcoming
instruments.
