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

TL;DR

This study uses JWST data to analyze the shift from stochastic to secular star formation in early galaxies around redshift 9, revealing a transition in galaxy behavior over cosmic time.

Contribution

It introduces a non-parametric SFH analysis with SFR gradients to characterize star formation modes across redshifts, highlighting a transition around z=9.

Findings

87% of massive galaxies at z>9 have stochastic SFHs

Fraction of galaxies with secular SFHs increases below z=7

Dispersion of UV magnitude distribution is about 1.2 mag

Abstract

Star formation histories (SFH) of early (6$<z<$12) galaxies have been found to be highly stochastic in both simulations and observations, while at $z\lesssim$6 the presence of a main sequence (MS) of star-forming galaxies imply secular processes at play. In this work, we aim at characterising the SFH variability of early galaxies as a function of their stellar mass and redshift. We use the JADES public catalogue and derive the physical properties of the galaxies as well as their SFH using the spectral energy distribution modelling code CIGALE. To this aim, we implement a non-parametric SFH with a flat prior allowing for as much stochasticity as possible. We use the SFR gradient, an indicator of the movement of galaxies on the SFR-$M_\ast$ 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_\ast/M_\odot)\gtrsim$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-$M_\ast$ 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 $z\gtrsim10$, by estimating $\sigma_{UV}$, the dispersion of the UV absolute magnitude distribution, to be of the order of 1.2mag and compare it with predictions from the literature. In conclusion, we find a transition of star-formation mode happening around $z\sim9$: Galaxies with stochastic SFHs dominates at $z\gtrsim9$, although this level of stochasticity is too low to reach those invoked by recent models to reproduce the observed UV luminosity function.