Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-08-09
Science 293:1112-1116,2001
Astronomy and Astrophysics
Astrophysics
8 pages, 4 PostScript figures, uses aaspp4.sty. Appearing in Science, Vol. 293, Issue 5532, August 10, 2001
Scientific paper
10.1126/science.1062693
The neutral hydrogen and the ionized helium 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 1000-1187 A band at a resolving power of 15,000. We resolve the He II Ly alpha 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 log N(HI) > 12.3 that account for most of the observed opacity in He II Ly alpha. The He II to H I column density ratio ranges from 1 to >1000 with an average of ~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 Ly alpha 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.
Cowie Lennox L.
Davidsen Arthur F.
Deharveng Jean Michel
Friedman Scott David
Giroux Mark Laird
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