Detection of intergalactic He II absorption at redshift 3.5

The large number of quasars found in the Sloan Digital Sky Survey has allowed searches for elusive, clear lines of sight toward He II Lyα absorption, a sensitive probe of the intergalactic medium. The few known systems indicate that He II reionization occurs at z > 3. We report the detection of a He II Lyα absorption edge in a quasar spectrum at z = 3.50, the most distant such feature found to date. The candidate quasar was selected from a z ∼ 3 sample in the Sloan Digital Sky Survey spectroscopic quasar survey and confirmed as part of an HST Space Telescope Imaging Spectrograph SNAP survey. We discuss the general characteristics of the absorption feature, as well as the probability for the discovery of additional such objects.