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Galaxies and their Circumgalactic Media at the Edge of Reionization

08-13-2020 - We will assess the hypothesis that faint galaxies drove cosmological hydrogen reionization to completion through detailed comparisons between observations of galaxies and their circumgalactic media (CGM) at the tail-end of the reionization epoch versus predictions from a new, accurate suite of cosmological radiation hydrodynamic calculations called the "Technicolor Dawn" simulations. Recent observational efforts to connect galaxies and their environments at the end of reionization offer a new opportunity to assess directly what role galaxies played in lifting the fog of cosmic dawn. While a consensus is emerging that low-mass galaxies may have dominated hydrogen reionization, this scenario invokes untested assumptions regarding both the abundance of very faint galaxies, and the nature of their stellar populations. A direct way to assess the galaxy-driven reionization hypothesis is to study the relationship between galaxies and their CGM, which is enriched with heavy elements shortly after the first stars form. In the last three years, observations have used metal absorption to track the signatures of faint galaxies out to z=7. Meanwhile, new integral field unit spectrographs and the James Webb Space Telescope will soon make followup surveys for absorbers' host galaxies routine. How will these observations constrain the abundance of faint galaxies and their stellar populations? An understanding of the relationship between early galaxies and their environments requires realistic models. Traditional models assume incorrectly that the intergalactic medium (IGM) and CGM are irradiated by a spatially-homogeneous ultraviolet background (UVB). By contrast, our simulations combine a predictive galaxy formation model with a spatially-resolved, multifrequency radiation transport solver, enabling them to model simultaneously the galaxies, their CGM, the UVB, and the progress of reionization. We will use detailed comparisons between our simulations and observations to address the two questions raised above through three closely-related inquiries.