G. A. Kriss,12*
J.
M. Shull,3
W. Oegerle,4
W. Zheng,2
A. F. Davidsen,2
A. Songaila,5
J. Tumlinson,3
L. L. Cowie,5
J.-M. Deharveng,6
S. D. Friedman,2
M. L. Giroux,3
R. F. Green,7
J. B. Hutchings,8
E. B. Jenkins,9
J. W. Kruk,2
H. W. Moos,2
D. C. Morton,8
K. R. Sembach,2
T. M. Tripp9
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 
(Ly) 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 NH
I > 1012.3 per square centimeter
that account for most of the observed opacity in He II
Ly. 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 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.
1 Space Telescope Science Institute, 3700 San
Martin Drive, Baltimore, MD 21218, USA.
2 Center for
Astrophysical Sciences, Department of Physics and Astronomy, Johns
Hopkins University, Baltimore, MD 21218, USA.
3 CASA
and JILA, Department of Astrophysical and Planetary Sciences,
University of Colorado, Campus Box 389, Boulder, CO 80309, USA.
4 Laboratory for Astronomy and Solar Physics, Code
681, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
5 Institute for Astronomy, University of Hawaii,
2680 Woodlawn Road, Honolulu, HI 96822, USA.
6 Laboratorie d'Astronomie Spatiale, BP 8, 13376 Marseille Cedex 12, France.
7 Kitt Peak National
Observatory, National Optical Astronomy Observatories, Post Office Box
26732, 950 North Cherry Avenue, Tucson, AZ 85726, USA.
8 Herzberg Institute of Astrophysics, National
Research Council of Canada, Victoria, BC V8X 4M6, Canada.
9 Princeton University Observatory, Princeton, NJ
08544, USA.
*
To whom correspondence should be addressed. E-mail:
gak{at}stsci.edu
Deceased.
