Stochastic star formation in early galaxies: JWST implications

The star formation rate (SFR) in high redshift galaxies is expected to be time-variable due to competing physical processes. Such stochastic variability might boost the luminosity of galaxies, possibly explaining the over-abundance seen at z 10 by JWST. We aim at quantifying the amplitude and timescales of such variability, and identifying the key driving physical processes. We select 245 $z=7.7$ galaxies with stellar mass 5× 10⁶ M_/ M_ 5× 10¹⁰ from SERRA, a suite of high-resolution, radiation-hydrodynamic cosmological simulations. After fitting the average SFR trend, SFR, we quantify the time-dependent variation, δ(t) ≡ log [ SFR/ SFR ] for each system, and perform a periodogram analysis to search for periodicity modulations. We find that $δ(t)$ is distributed as a zero-mean Gaussian, with standard deviation σ_δ 0.24 (corresponding to a UV magnitude s.d. σUV 0.61) that is independent of M_. However, the modulation timescale increases with stellar mass: t_δ~ (9, 50, 100)\, Myr for M_ ~ (0.1, 1, 5)× 10⁹\, M_, respectively. These timescales are imprinted on the SFR by different processes: (i) photoevaporation, (ii) supernova explosions, and (iii) cosmological accretion/merging dominating in low, intermediate, and high mass systems, respectively. The predicted SFR variations cannot account for the required z 10 UV luminosity function boost. Other processes, such as radiation-driven outflows clearing the dust, must then be invoked to explain the enhanced luminosity of super-early systems.