2008ApJ...686..195Z

We present the UV spectra of two quasars obtained with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) 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 Å of τ₃₀₄>3.1 (90% confidence), or a He III 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 II 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/cm2. 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 II and H I signatures are consistent with photoionization by a compact (1-100 pc) star-forming region located approximately 10 Mpc from the quasar.