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Intergalactic He II Absorption in the Spectra of Quasars at Redshifts 3.5 and 3.8, Observed with the HST ACS Prism

Research output: Contribution to journalArticle

  • Wei Zheng
  • Avery Meiksin
  • Keith Pifko
  • Scott F. Anderson
  • Craig J. Hogan
  • Eric Tittley
  • Gerard A. Kriss
  • Kuenley Chiu
  • Donald P. Schneider
  • Donald G. York
  • David H. Weinberg

Related Edinburgh Organisations

Original languageEnglish
Pages (from-to)195-200
Number of pages6
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 2008


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.

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