How the first generations of luminous baryons established the X-ray and   UV backgrounds
  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. 
Andrei Mesinger
astro-ph.CO