LYRA. III. The Smallest Reionization Survivors

Abstract The dividing line between galaxies that are quenched by reionization (“relics”) and survive (i.e., continue forming stars) is commonly discussed in terms of a halo mass threshold. We probe this threshold physically more complete accurate way than has been possible to date, using five extremely high resolution ( M target = 4 ⊙ ) cosmological zoom-in simulations dwarf within the range (1–4) × 10 9 . employed LYRA simulation model features resolved interstellar medium physics individual, supernova explosions. Interestingly, two out simulated lie close but neither full relics nor survivors. These initially quench at time merely remain quiescent for ∼500 Myr. At z ∼ 5 they recommence star formation synchronous star-forming until present day. parallel timing indicates consistent sound-crossing cooling times halos. While histories we find diverse, show directly related ability given retain cool gas. Whereas latter most strongly dependent on (or virial temperature) host reionization, it also depends its growth history, UV background (and decrease late times), amount metals retained halo.