Intergalactic HeiiAbsorption in the Spectra of Quasars at Redshifts 3.5 and 3.8, Observed with theHSTACS Prism

We present the UV spectra of two quasars obtained with the Hubble Space Telescope Advanced Camera for Sur- veys (HSTACS) prism (R 100). One quasar, at redshift z ¼ 3:5, shows an absorption edge consistent either with nearlyfullyionizedheliumwithanintergalacticHeiiLyopticaldepthat3048of 304 > 3:1(90%confidence),ora Heiiiregion with edge 5.4 Mpc from the QSO. The spectrum of the second quasar, atz ¼ 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 10 22 cm 2 . In this case, the extremely high He iicolumn density and absence of strong H iabsorption 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 (1Y100 pc) star-forming region located approximately 10 Mpc from the quasar. Subject headinggs: dark matter — intergalactic medium — quasars: individual (SDSS J1711+6052, SDSS J2346-0016) — surveys — ultraviolet: general