Gabor Worseck (Potsdam University)

The Epoch of Helium Reionization - The Final Phase Transition of the Universe
When Feb 14, 2019 from 02:30 PM to 03:30 PM
What
  • Colloquium
Where SH Lecture Hall
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Akin to neutral hydrogen, intergalactic singly ionized helium can be probed by Lyman alpha forest spectroscopy of quasars. The advent of GALEX and HST/COS have revolutionized our view of HeII reionization, the final major phase transition of the intergalactic medium. Efficient surveys pioneered by my team densely sample the strongly fluctuating HeII absorption at 2.7<z<3, evoking a picture of overlapping HeIII bubbles around quasars at the end of HeII reionization at z~2.7. Unexpectedly, the seven z>3.5 sightlines observed to date show some regions with non-zero transmission that suggest HeII reionization was well underway by z=4. This is in tension with predictions from current radiative transfer simulations of a HeII reionization driven by bright quasars. To explore this further we have performed a comprehensive reanalysis of the quasar UV luminosity function, finding a factor ~2 higher quasar emissivity at z>3 than employed in conventional HeII reionization and UV background synthesis models. Our higher quasar space density remains insufficient to drive hydrogen reionization, but may reproduce the high and mildly fluctuating ionization state of helium at z>3.