Resolving the Structure of Ionized Helium in the Intergalactic Medium with the Far Ultraviolet Spectroscopic Explorer

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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 α (Lyα) 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 Lyα. The He ii to H i column density ratio ranges from 1 to >1000, with an average of ∼80. Ratios of 100 in many locations indicate additional contributions from starburst galaxies or heavily filtered quasar radiation. The presence of He ii Lyα 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.
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