Cosmic Sands: The Origin of Dusty, Star-forming Galaxies in the Epoch of Reionization

TL;DR

This paper introduces the Cosmic Sands simulations to study the formation of the first massive, dusty galaxies during the epoch of reionization, revealing their growth through mergers, gas accretion, and early dust production.

Contribution

It presents a new suite of cosmological zoom-in simulations that model the early buildup of massive, dusty galaxies, incorporating detailed galaxy formation physics and dust radiative transfer.

Findings

Massive proto-galaxies undergo frequent mergers boosting star formation.

Galaxies are chemically evolved and contain dust masses comparable to observations at z=10.

High SFRs are sustained mainly by smooth gas accretion, with major mergers triggering starbursts.

Abstract

We present the Cosmic Sands suite of cosmological zoom-in simulations based on the Simba galaxy formation model in order to study the build up 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 build up of stellar mass. The galaxies are already appreciably chemically evolved by z=10, with modeled dust masses comparable to those inferred from observations in the same epoch. 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$_{\odot}$ / yr 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 radiative transfer, 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 sub-millimeter 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.