Mini-Quenching of High-Redshift Galaxies by Bursty Star Formation

The recent observation of a low-mass $z=5.2$ and an intermediate-mass $z=7.3$ (JADES-GS-z7-01-QU) quenched galaxy with JWST / NIRSpec is the first evidence of halted star formation above $z\sim 5$. Here we show how bursty star formation at high redshift gives rise to temporarily quenched, or miniquenched galaxies in the mass range $M_{\star} = 10^7-10^9 \ M_{\odot}$ using three models of galaxy formation: the periodic box simulation IllustrisTNG, the zoom-in simulation VELA and an empirical halo model. The main causes for mini-quenching are stellar feedback, lack of gas accretion onto galaxies and galaxy-galaxy interactions. The abundance of mini-quenching events agrees across the three models: the population first appears below $z\sim 8$, after which the fraction of miniquenched galaxies increases with cosmic time, from $\sim 0.5$% at $z=7$ to $\sim 1-2$% at $z=4$, corresponding to comoving number densities of $8.0\times 10^{-6}$ Mpc$^{-3}$ and $5.4\times 10^{-4}$ Mpc$^{-3}$, respectively. The star formation rate duty cycle ($f_{\mathrm{duty}}\sim 99.56^{+0.4}_{-4.5}$% at $z=7$) inferred for VELA galaxies is consistent therewith. Star formation histories (SFHs) in VELA suggest that mini-quenching at $z=4-8$ is short-lived with a duration of $\sim 20-40$ Myr, which is close to the free-fall timescale of the inner halo. However, mock spectral energy distributions of miniquenched galaxies in IllustrisTNG and VELA do not match JADES-GS-z7-01-QU photometry, unless their SFHs are artificially altered to be more bursty on timescales of $\sim 40$ Myr. Studying miniquenched galaxies might aid in calibrating the sub-grid models governing galaxy formation, as these may not generate sufficient burstiness at high redshift to explain the SFH inferred for JADES-GS-z7-01-QU.