Like a candle in the wind: the embers of once aflame, now smouldering galaxies at 5 < z < 8

We develop a photometric search method for identifying smouldering galaxies at 5 < z < 8, which are defined to have weak emission lines and thus generally have low specific star formation rates and may even be in a state of (temporary) quiescence. The deep NIRCam imaging (∼29.5 AB mag, 5σ) from the JADES second data release is essential for finding these systems, as they are faint, relatively quiescent dwarf galaxies (M_∗ ∼ 10^8–9 M) in the Epoch of Reionization (EoR). Moreover, medium band imaging is key, enabling a clear identification of the lack of emission lines in these galaxies, thus betraying their dormant flame. Owing to the young age of the Universe, combined with the likely bursty star formation in these first dwarf galaxies, conventional colour-selection methods like the UVJ diagram likely miss a large fraction of the quiescent population in the EoR. Indeed, we find that smouldering galaxies constitute a considerable fraction (0.05–0.35) of the EoR dwarf galaxy population (M_∗ ∼ 10^8–9 M). As predicted by simulations, these first dwarf galaxies are fragile, the star formation in their shallow potential wells easily snuffed out by feedback-driven winds triggered by secular or merger-driven starbursts, with the smouldering fraction increasing with decreasing stellar mass. Finally, we provide observational constraints on the smouldering galaxy comoving number density (∼10⁻⁴–10⁻⁵ dex⁻¹ Mpc⁻³), which, although hampered by incompleteness, should aid in our understanding of the primordial baryon cycle, as current simulations greatly disagree on whether these systems are rare (∼1 per cent) or common (∼50 per cent) in the EoR.