Progress in studies of intergalactic He II absorption

Significant progress has been made in the last decade toward an understanding of the intergalactic He II absorption. At redshifts z < 2.8, individual absorbers are found to be ionised mainly by a hard quasar continuum, but their ionisation states vary considerably and the ionising radiation field may have contributions from a soft stellar continuum. The He II opacity increases significantly at z ∼ 2.8, suggesting a possible end of the general reionisation of intergalactic He II. Using the massive SDSS database and UV selection with HST and GALEX, we have found new quasars at z ≥ 3.5, with detected He II absorption. Future observations will pinpoint the epoch of reionisation of the intergalactic He II.