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Quasars and the Ionization of the Intergalactic Medium: Helium Reionization and Beyond
David Syphers (Eastern Washington University)
November 10, 2016 @ 4:00 PM - 5:00 PM
For most of the history of the universe, quasars have been the dominant source of ionizing photons. Most of the baryons in the universe reside in the intergalactic medium (IGM), even at the present. The most dramatic interaction between these two was the last major phase transition of the universe, when quasars fully reionized helium at z~3. I will discuss our current observational understanding of the helium reionization epoch, as well as what these observations reveal about quasars themselves in the extreme UV (EUV). The EUV quasar spectrum can be modeled as a power law, but the average spectral index and its dispersion are poorly understood, leading to large uncertainty in theoretical models of IGM ionization. Restframe EUV observations with HST are allowing us to finally constrain quasar continuum emission in the EUV, and disentangle it from weak emission lines.