How the first generations of luminous baryons established the X-ray and UV backgrounds

TL;DR

This paper explores how early luminous baryons influenced the X-ray and UV backgrounds, affecting the epoch of reionization and the evolution of the intergalactic medium, with insights into future 21cm observational prospects.

Contribution

It provides a theoretical framework for understanding the impact of early baryonic sources on reionization and discusses how upcoming 21cm observations can clarify unresolved questions.

Findings

Early X-ray sources heated the IGM rapidly.

Reionization was driven by atomically-cooled galaxies, less affected by feedback.

Final reionization stages involved LLS photo-evaporation.

Abstract

The first generations of astrophysical objects made a substantial impact on our Universe with their radiation. X-rays from the first sources, with large mean free paths, likely quickly heated the intergalactic medium (IGM). The second generation of 21cm instruments can provide a unique view into this early epoch. The early stages of reionization likely followed, driven by so-called "minihalos", i.e. molecularly-cooled halos. These small halos were susceptible to complex feedback mechanisms, especially from the soft-UV background which preceded reionization, resulting in complex and possibly extended early stages of reionization. When atomically-cooled galaxies emerged as the dominant ionizers, reionization could proceed more rapidly, with these being less sensitive to radiative feedback than previously thought. Reionization could have slowed in the final stages when the ionized bubbles grew larger than the separation of Lyman limit absorption systems (LLSs). The final stages likely involved the photo-evaporation of LLSs, which by then regulated the rise of the UV background. I discuss the theoretical underpinnings of this narrative, as well as how future 21cm observations may help shed light on the outstanding uncertainties.