Tobias Schmidt

Tobias Schmidt (Germany)

tschmidt @

The HeII Transverse Proximity Effect

Abstract: The reionization of Helium at z~3 marks the last cosmological phase change in the intergalactic medium (IGM) and is expected to heat the IGM and harden the the UV background. One of the great advantages of Helium reionization is that we can simultaneously observe the sources which reionize helium, namely luminous quasars, and study their impact on
reionization. During the last few years, the community was able to collect science grade HST/COS spectra for nearly all ~20 far UV HeII transparent quasars and our team is now conducting a dedicated survey to find foreground quasars close to these sightlines. This dataset allows us to characterize the impact of the foreground quasar ionizing radiation on the Helium optical depth in the background sightline and search for the so called 'transverse proximity effect' of the foreground quasars.
Based on a large sample of ~15 foreground quasars, I will present the first robust statistical detection of the Helium transverse proximity effect, which was up to now unambiguously detected only for one prototype object. Then I will describe our efforts to model this measurement by synthesizing the UV background field using the locations of foreground quasars and discuss the dependencies on the neutral fraction of the IGM and on quasar properties, in particular the so far poorly constrained quasar lifetime.

Supervisor: Joe Hennawi (MPIA)

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