Probing the Epoch of Reionization with the Brightest High-Redshift Quasars

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An important period in the early history of the universe is the so-called "epoch of reionization". This is a period a few hundred million years after the Big Bang during which all of the atomic Hydrogen gas in the universe was ionized by newly formed massive stars. These were the first stars to form in the universe. Understanding this early period is one of the main goals of astrophysics. This is because properties of the unverse (and the objects causing the ionization) carry implications for models and simulations of how galaxies form and the physics of the matter being ionized (the intergalactic medium or IGM). This project will shed new light on reionization and high-redshift galaxies by observing the IGM after the epoch of reionization. At these times, the IGM contains a large amount of information on when and how reionization occurred. Fortunately, in the past year a set of "ultra-bright" quasars have been discovered that are much brighter than previously known quasars at similar distances. The PI and his team will make entirely new or greatly improved measurements of three aspects of the post-reionization IGM: (i) the smoothing of baryons in the IGM due to heating during reionization, (ii) the thermal changes of the IGM due to cool-down following reionization, and (iii) the most sensitive study to date of the enrichment of elements heavier than helium of gas in galaxies from this period. This latter measurement will aid in the understanding of early-period star formation.Education efforts supported by this project will reach young children in predominantly Hispanic, low-income communities. The PI and graduate student funded by this grant will help to develop, manage, and perform astronomy-themed workshops designed to promote STEM education in kindergarten, first, and second-grade students. The workshops will address California's Common Core State Standards for science, and target underserved communities in Riverside and surrounding areas. Workshop themes will include Touching the Universe, The Spiral Universe, Constellations Everywhere, and Smells of the Universe. This project clearly relates to NSF's mission to promote the progress of science. In addition, the "Broader Impact" portion of the project does this, and advances the national health, prosperity and welfare by helping to develop interest in, and educating, STEM activities among the next generation of diverse individuals.Understanding the epoch of reionization remains one of the forefront goals of astrophysics. Along with being the last global phase change of the Universe, reionization carries key implications for models of galaxy formation and the physics of the intergalactic medium (IGM). Over the past several years, notable advances have been made through observations of high-redshift galaxies, quasars, the cosmic microwave background, and other probes. Currently, however, the combined constraints on when reionization occurred remain relatively weak, with the bulk of observations being consistent with reionization ending either relatively late (z~6-7) or significantly earlier (z~8-9).Motivated by these new objects, the PI and his team will make entirely new or greatly improved measurements of three aspects of the post-reionization IGM and galaxy-IGM interface: (i) Measuring the Jeans smoothing of the IGM at z~5 will reveal how much energy has been injected at higher redshifts via photo-heating, and hence when reionization occurred (more total heating = earlier reionization). This measurement will constrain the characteristic redshift of reionization to within Deltazreion=+/-0.7, or a factor of two better than current CMB constraints. (ii) Measuring the temperature of the IGM out to the highest redshifts ever (z=5.7) will determine whether the IGM is still warm at that epoch from having recently been reionized. Alternatively, an IGM that has already cooled by z~5.5 will imply that reionization largely occurred at z7.5. (iii) Finally, the survey for CIV at z~5 will be sensitive to metals down to the cosmic mean density, probing a factor of six more volume than any other direct survey at any redshift. This will reveal, for the first time, the full extent to which reionization-era galaxies enriched their surroundings via galactic winds. Searching for weak O I systems at z~6 also will establish whether the drop in C IV with redshift at z5 is due to a decline in metallicity or evolution of the UV background.