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 σδ ≃ 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: tδ ∼ (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....