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, 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.