Stochastic star formation in early galaxies: JWST implications

Abstract

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 $z\gtrsim 10$ 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 $5\times 10^6\lesssim M_{\star }/{\mathrm{M}}_{\odot }\lesssim 5\times 10^{10}$ from SERRA, a suite of high-resolution, radiation-hydrodynamic cosmological simulations. After fitting the average SFR trend, $⟨\mathrm{SFR}⟩$, we quantify the time-dependent variation, $\delta (t)\equiv \log [SFR/⟨\mathrm{SFR}⟩]$ for each system, and perform a periodogram analysis to search for periodicity modulations. We find that $\delta (t)$ is distributed as a zero-mean Gaussian, with standard deviation ${\sigma }_{\delta }\simeq 0.24$ (corresponding to a UV magnitude s.d. ${\sigma }_{\mathrm{UV}}\simeq 0.61$) that is independent of $M_{\star }$. However, the modulation timescale increases with stellar mass: $t_{\delta }\sim (9,50,100)\mathrm{Myr}$ for $M_{\star }\sim (0.1,1,5)\times 10^9{\mathrm{M}}_{\odot }$, 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 $z\gtrsim 10$ 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.

5 pages, 4 figures, accepted by A&A

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