Igniting galaxy formation in the post-reionization universe

TL;DR

This study uses cosmological simulations to identify key factors like cold-dense gas and halo concentration that trigger galaxy formation after reionization, revealing that ignition can occur as late as redshift 2.

Contribution

It demonstrates the mechanisms behind galaxy ignition post-reionization using FIRE-2 simulations, highlighting the roles of cold-dense gas and halo concentration.

Findings

100% of recently-ignited halos have cold-dense gas enhancements

83% show enhancements in both cold-dense gas and halo concentration

Galaxy ignition can occur as late as redshift 2

Abstract

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 a 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 (FIRE-2) physics model, to characterize the mechanisms governing galaxy ignition in the post-reionization 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 help 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 ($c_{\rm NFW}$), while the remaining $\sim17\%$ exhibit enhanced cold-dense gas content and suppressed $c_{\rm NFW}$ 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.