Quiescent Low-mass Galaxies Observed by JWST in the Epoch of Reionization

The surprising JWST discovery of a quiescent, low-mass (M ⋆ = 108.7 M ⊙) galaxy at redshift z = 7.3 (JADES-GS-z7-01-QU) represents a unique opportunity to study the imprint of feedback processes on early galaxy evolution. We build a sample of 130 low-mass (M ⋆ ≲ 109.5 M ⊙) galaxies from the SERRA cosmological zoom-in simulations, which show a feedback-regulated, bursty star formation history (SFH). The fraction of time spent in an active phase increases with the stellar mass from f duty ≈ 0.6 at M ⋆ ≈ 107.5 M ⊙ to ≈0.99 at M ⋆ ≥ 109 M ⊙, and it is in agreement with the value f duty ≈ 0.75 estimated for JADES-GS-z7-01-QU. On average, 30% of the galaxies are quiescent in the range 6 < z < 8.4; they become the dominant population at M ⋆ ≲ 108.3 M ⊙. However, none of these quiescent systems matches the spectral energy distribution of JADES-GS-z7-01-QU, unless their SFH is artificially truncated a few Myr after the main star formation peak. As supernova feedback can only act on a longer timescale (≳30 Myr), this implies that the observed abrupt quenching must be caused by a faster physical mechanism, such as radiation-driven winds from young massive stars and/or an active galactic nucleus.