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03/28/19: SO/NOAO Joint Colloquium Series: Steven Finkelstein, University of Texas


Title: Implications of Reionizing the Universe with Low Galaxy Escape Fractions

The reionization of the intergalactic medium (IGM) was the last major phase transition of the universe, and both the time evolution and spatial variation of this process encode key information about the onset of luminous objects in the universe. While we think that massive stars within star-forming galaxies provide the needed ionizing photons, the observed escape fractions of these photons are too low to complete reionization when combined with typical assumptions. We have devised a new semi-empirical model, utilizing simulation-predicted escape fractions (where only the smallest halos have large escape fractions) in combination with observed galaxy luminosity functions. Using physically motivated priors on the threshold for star-formation based on halo mass rather than a single limiting magnitude, and allowing the ionizing photon production efficiency to evolve as suggested by observations, we find that it is possible for reionization to be completed by z=5.5 with low (<5%) average ionizing photon escape fractions. Our model makes a number of testable predictions, including: 1) AGNs contribute non-negligibly to the end of reionization, 2) the neutral fraction at z~7 is only 20%, and 3) significant star-formation must be occurring at z~9-10. I will show observational results from my group at UT Austin testing all of these assumptions, including results from ultra-deep Keck spectroscopy for Lyman-alpha emission at z~7-10, and the discovery of several remarkably bright galaxy candidates at z > 9. I will finish by discussing how early observations from JWST will further test these predictions, leading to a much better understanding of how early galaxies formed, and began the reionization process.

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