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G. A. Kriss (STScI), FUSE IGM Working Group Team
The neutral hydrogen and ionized helium absorption in the spectra of quasars probe the structure of intergalactic gas in the early universe. At redshifts above 3.5, the universe is opaque to He II absorption. At lower redshifts, this opacity decreases as the extragalactic background radiation gradually ionizes the He II gas. At redshifts of ~3, transparent holes become visible, and by z~2.7, the universe is largely transparent in the light of He II Lyman \alpha. Using FUSE, we have resolved the He II Lyman \alpha forest along the lines of sight to two different quasars, HE2347--4342 and HS1700+64. By comparing the opacity in He II Lyman forest features to corresponding features in the H I Lyman forest obtained from deep ground-based observations, we find that the typical radiation source for the extragalactic background is hard, just like the light from quasars. This radiation field is far from uniform, however. It fluctuates in spectral index on spatial scales of ~1 Mpc, possibly responding to fluctuations in the extragalactic source population. The population of ionizing sources is dominated by quasars or AGN, but many soft sources with spectra similar to star forming galaxies contribute to some observed features.
This work is based on data obtained by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University. Financial support to U.S. participants has been provided by a contract from NASA.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.