Mini-quenching of z = 4–8 galaxies by bursty star formation

Abstract

The recent reported discovery of a low-mass z = 5.2 and an intermediate-mass z = 7.3 quenched galaxy with JWST/NIRSpec is the first evidence of halted star formation above z ≈ 5. Here, we show how bursty star formation at z = 4–8 gives rise to temporarily quenched, or mini-quenched galaxies in the mass range M* = 107–109 M☉ using four models of galaxy formation: the periodic box simulation ILLUSTRISTNG, the zoom-in simulations VELA and FIRSTLIGHT 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-)quenched galaxies agrees across the models: the population first appears below z ≈ 8, after which their proportion increases with cosmic time, from ∼0.5–1.0 per cent at z = 7 to ∼2–4 per cent at z = 4, corresponding to comoving number densities of ∼10−5 and ∼10−3 Mpc−3, respectively. These numbers are consistent with star formation rate duty cycles inferred for VELA and FIRSTLIGHT galaxies. Their star formation histories (SFHs) suggest that mini-quenching at z = 4–8 is short-lived with a duration of ∼20–40 Myr, which is close to the free-fall time-scale of the inner halo. However, mock spectral energy distributions of mini-quenched 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 time-scales of ∼40 Myr. Studying mini-quenched galaxies might aid in calibrating 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.