Stochastic star formation in early galaxies: Implications for the James Webb Space Telescope

Contact. The star formation rate (SFR) in high-redshift galaxies is expected due to competing physical processes. This stochastic variability might boost the luminosity of galaxies and might explain the over-abundance seen at z ≳ 10 by the James Webb Space Telescope.
Aims: We quantify the amplitude and timescales of this variability and identify the key physical processes.
Methods: We selected 245 z = 7.7 galaxies with stellar mass 5 × 10⁶ ≲ M_⋆/M_⊙ ≲ 5 × 10¹⁰ from SERRA, which is a suite of high-resolution radiation-hydrodynamic cosmological simulations. After fitting the average SFR trend, ⟨SFR⟩, we quantified the time-dependent variation, δ(t)≡log[SFR/⟨SFR⟩], for each system and performed a periodogram analysis to search for periodicity modulations.
Results: 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. σ_UV ≃ 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.
Conclusions: 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.