Astraeus IV: Quantifying the star foation histories of galaxies in the Epoch of Reionization

We use the astraeus framework, which couples an N-body silation with a semi-analytic model for galaxy foation and a semi-numerical model for reionization, to quantify the star foation 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 log(SFR($z$)) = -α(1 + $z$) + β; star foation is deemed stochastic if it deviates from this fit by more than $Delta _math{SFR}=0.6,$dex. Our key findings are as follows: (i) The fraction of stellar mass foed and time spent in the stochastic phase decrease with increasing stellar mass and redshift $z$. While galaxies with stellar masses of $M_star 107 M_odot$ at $z$ 5 (10) fo $70(20)$ of their stellar mass in the stochastic phase, this reduces to $lt 10$ at all redshifts for galaxies with $M_star gt 10{10} M_odot$; (ii) the fractional mass assembled and lifetime spent in the stochastic phase do not significantly change with the radiative feedback model used; and (iii) at all redshifts, α increases (decreases for the strongest radiative feedback model) with stellar mass for galaxies with $M_star less10{8.5} M_odot$ and converges to 0.18 for more massive galaxies; β always increases with stellar mass. Our proposed fits can reliably recover the stellar masses and mass-to-light ratios for galaxies with $M_star 10{8}10{10.5}, M_odot$ and ${it M}_{ UV}-17{ to}-23$ at $z$ 5-9. This physical model can therefore be used to derive the SFHs for galaxies observed by a number of forthcoming instruments.