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 (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 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 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.
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs GO 9759, 9806 and 10231.