Intergalactic He II absorption in the spectra of quasars at redshifts 3.5 and 3.8, observed with the HST ACS prism

We present the UV spectra of two quasars obtained with the Hubble Space Telescope Advanced Camera for Surveys (HSTACS) prism (R ∼ 100). One quasar, at redshift z = 3.5, shows an absorption edge consistent either with nearly fully ionized helium with an intergalactic He II Lyα optical depth at 304 Åof τ₃₀₄ > 3.1 (90% confidence), or a He m region with edge 5.4 Mpc from the QSO. The spectrum of the second quasar, at z = 3.8, has a broad absorption profile extending across the He n emission line of the quasar consistent with a damped absorption profile from an intervening He II system at z ∼ 3.7 with a column density of approximately 10²² cm ^-2. In this case, the extremely high He II column density and absence of strong H I absorption suggests that the radiation field is quite weak at 4 ryd. The combined He n and H I signatures are consistent with photoionization by a compact (1-100 pc) star-forming region located approximately 10 Mpc from the quasar.