Intergalactic He II Absorption in the Spectra of Quasars at Redshifts 3.5 and 3.8, Observed with the HST ACS Prism
Anderson, Scott F.; Schneider, Donald P.; York, Donald G.; Kriss, Gerard A.; Zheng, Wei; Weinberg, David H.; Meiksin, Avery; Pifko, Keith; Hogan, Craig J.; Tittley, Eric; Chiu, Kuenley
United States, United Kingdom, Australia
Abstract
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 zsimeq 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.