Journal Article

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

Cowie, L. L., Davidsen, A. F., Kriss, G. A., Oegerle, W., Songaila, A., Zheng, W., Friedman, S. D., Green, R. F., Shull, J. M., Kruk, J. W., Jenkins, E. B., Hutchings, J. B., Tumlinson, J., Sembach, K. R., Morton, D. C., Moos, H. W., Giroux, M. L., J.-M. Deharveng, Tripp, T. M.

2001

Overview

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 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 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.

• Publisher: American Society for the Advancement of Science,American Association for the Advancement of Science,The American Association for the Advancement of Science
• Subject: Absorption and adsorption / Absorption spectra / Astronomy / Astrophysics / Characteristics and properties of external galaxies and extragalactic objects / Earth, ocean, space / Exact sciences and technology / Galaxies / Gas absorption / Gas adsorption / Gases / Helium / Hubble Space Telescope / Hydrogen / Intergalactic matter ; quasar absorption and emission-line systems ; lyman forest / Intergalactic medium / Light / Line spectra / Observations / Opacity / Quasars / Radiation / Red shift / Spectral index / Stars & galaxies / Stellar systems. Galactic and extragalactic objects and systems. The universe / Ultraviolet radiation / Wavelengths