title: Resolving the structure of ionized helium in the intergalactic medium with the far ultraviolet spectroscopic explorer.
abstract: The neutral hydrogen (H I) and ionized helium (He II) absorption in the spectra of quasars are unique probes of structure in the early universe. We present Far-Ultraviolet Spectroscopic Explorer observations of the line of sight to the quasar HE2347-4342 in the 1000 to 1187 angstrom band at a resolving power of 15,000. We resolve the He II Lyman alpha (Lyalpha) absorption as a discrete forest of absorption lines in the redshift range 2.3 to 2.7. About 50 percent of these features have H I counterparts with column densities N(H I) > 10(12.3) per square centimeter that account for most of the observed opacity in He II Lyalpha. The He II to H I column density ratio ranges from 1 to >1000, with an average of approximately 80. Ratios of <100 are consistent with photoionization of the absorbing gas by a hard ionizing spectrum resulting from the integrated light of quasars, but ratios of >100 in many locations indicate additional contributions from starburst galaxies or heavily filtered quasar radiation. The presence of He II Lyalpha absorbers with no H I counterparts indicates that structure is present even in low-density regions, consistent with theoretical predictions of structure formation through gravitational instability.
authors.affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA. gak@stsci.edu
authors.forename: G A
authors.initials: GA
authors.lastname: Kriss
authors.forename: J M
authors.initials: JM
authors.lastname: Shull
authors.forename: W
authors.initials: W
authors.lastname: Oegerle
authors.forename: W
authors.initials: W
authors.lastname: Zheng
authors.forename: A F
authors.initials: AF
authors.lastname: Davidsen
authors.forename: A
authors.initials: A
authors.lastname: Songaila
authors.forename: J
authors.initials: J
authors.lastname: Tumlinson
authors.forename: L L
authors.initials: LL
authors.lastname: Cowie
authors.forename: J M
authors.initials: JM
authors.lastname: Deharveng
authors.forename: S D
authors.initials: SD
authors.lastname: Friedman
authors.forename: M L
authors.initials: ML
authors.lastname: Giroux
authors.forename: R F
authors.initials: RF
authors.lastname: Green
authors.forename: J B
authors.initials: JB
authors.lastname: Hutchings
authors.forename: E B
authors.initials: EB
authors.lastname: Jenkins
authors.forename: J W
authors.initials: JW
authors.lastname: Kruk
authors.forename: H W
authors.initials: HW
authors.lastname: Moos
authors.forename: D C
authors.initials: DC
authors.lastname: Morton
authors.forename: K R
authors.initials: KR
authors.lastname: Sembach
authors.forename: T M
authors.initials: TM
authors.lastname: Tripp