Cosmic sands: the origin of dusty, star-forming galaxies in the epoch of reionization

Abstract

We present the Cosmic Sands suite of cosmological zoom-in simulations based on the simba galaxy formation model in order to study the buildup of the first massive and dusty galaxies in the early universe. Residing in the most massive halos, we find that the compact proto-massive galaxies undergo nearly continuous mergers with smaller subhalos, boosting star formation rates (SFRs) and the buildup of stellar mass. The galaxies are already appreciably chemically evolved by z = 7, with modeled dust masses comparable to those inferred from observations in the same epoch, except for the most extreme systems. We track gas accretion onto the galaxies to understand how extreme SFRs can be sustained by these early systems. We find that smooth gas accretion can maintain SFRs above 250 M · yr-1, but to achieve SFRs that boost galaxies well above the main sequence, a larger perturbation like a gas-rich major merger is necessary to trigger a starburst episode. Post-processing the Cosmic Sands simulations with dust RT, we find that, while the infrared luminosities of the most-dust-rich galaxies are comparable to local ULIRGs, they are substantially dimmer than classical z = 2 submillimeter galaxies. We end with a discussion on the possible reasons for this discrepancy at the highest masses and the future work we intend to carry out to study the chemical enrichment of the earliest dusty galaxies.