Speaker
Dr
Gabor Worseck
(Max-Planck-Institut für Astronomie)
Description
The advent of GALEX and HST/COS have revolutionized studies of HeII reionization. In HST Cycle 17 we pioneered an effective strategy to statistically probe the HeII reionization epoch with HST/COS spectroscopy of the UV-brightest quasars. The clear picture emerging from the 13 sightlines probing z<3 is that HeII reionization likely ended at z~2.7. At z>2.7 the large scatter in the mean HeII absorption on scales of ~10 proper Mpc indicates that HeII reionization was highly inhomogeneous and extended. Complementary Keck+VLT spectroscopy of the coeval HI forest maps the underlying density field along the quasar sightline, yielding the first tantalizing evidence for "inside-out" HeII reionization occurring first in overdense regions around quasars. Some of these HeII-reionizing quasars have been identified by us in a dedicated deep imaging and spectroscopic survey for foreground quasars close to HeII sightlines. Currently, we are performing an HST/COS survey for HeII absorption toward 7 newly discovered UV-bright z>3.1 quasars. By doubling the redshift pathlength at z>3 we will obtain the first statistical insight into the early stages of HeII reionization.
