2026ApJ...997..181M

It is widely believed that the ultraviolet background produced during the epoch of reionization conspires against the formation of low-mass galaxies. Indeed, this mechanism is often invoked as part of the solution to the so-called “missing satellites problem.” In this paper we employ FIREbox, a large-volume cosmological simulation based on the Feedback In Realistic Environments physics model, to characterize the mechanisms governing galaxy ignition in the postreionization era. By carefully matching recently ignited halos (with stellar ages below 100 Myr at the time of selection) to halos that failed to form any stars, we conclude that the presence of cold dense gas and halo concentration helps incite the process of galaxy formation. Concretely, we find that 100% of recently ignited halos experience cold dense gas enhancements relative to their matched failed counterparts. Likewise, approximately 83% display enhancements in both cold dense gas and Navarro-Frenk-White concentration (c_NFW), while the remaining ∼17% exhibit enhanced cold dense gas content and suppressed c_NFW values. Lastly, our simulation suggests that galaxy ignition can occur as late as z = 2, potentially allowing us to observationally catch this process “in the act” in the foreseeable future.