Identification of a transition from stochastic to secular star formation around z = 9 with JWST

Abstract

Star formation histories (SFHs) of early galaxies (6 < z < 12) have been found to be highly stochastic in both simulations and observations, while at z≲6 the presence of a main sequence (MS) of star-forming galaxies implies secular processes at play. In this work we characterise the SFH variability of early galaxies as a function of their stellar mass and redshift. We used the JADES public catalogue and derived the physical properties of the galaxies as well as their SFHs using the spectral energy distribution modelling code CIGALE. To this end, we implemented a non-parametric SFH with a flat prior allowing for as much stochasticity as possible. We used the star formation rate (SFR) gradient, an indicator of the movement of galaxies on the SFR–M* plane, linked to the recent SFH of galaxies. This dynamical approach of the relation between the SFR and stellar mass allows us to show that, at z > 9, 87% of massive galaxies (log(M*/M⊙)≳9) have SFR gradients consistent with a stochastic star formation activity during the last 100 Myr, while this fraction drops to 15% at z < 7. On the other hand, we see an increasing fraction of galaxies with a star formation activity following a common stream on the SFR–M* plane with cosmic time, indicating that a secular mode of star formation is emerging. We place our results in the context of the observed excess of UV emission as probed by the UV luminosity function at z ≳ 10 by estimating σUV, the dispersion of the UV absolute magnitude distribution, to be of the order of 1.2 mag, and compare it with predictions from the literature. In conclusion, we find a transition of star formation mode happening around z ∼ 9: Galaxies with stochastic SFHs dominate at z ≳ 9, although this level of stochasticity is too low to reach those invoked by recent models to reproduce the observed UV luminosity function.