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Since the IGM turns out to be hot and therefore highly ionized

Since the IGM turns out to be hot and therefore highly ionized

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Since the IGM turns out to be hot and therefore highly ionized, with heating sources such as QSOs and star-forming galaxies common in the early Universe, searches have also been made for the He II continuous absorption at high redshifts, since this should be seen for plasmas in which the hydrogen is completely ionized (and therefore produces no ionization edge). Because it may be the dominant ionization stage of helium, and its reasonably large cross-section, He II is in a sense a stronger absorption target than hydrogen in the IGM. This test must be done for high-redshift objects, since the ionization energy goes as Z 2 x 13.6 eV; so we must see wavelengths shorter than 228 Å in the QSO emitted frame. Very few QSOs will be suitable background sources for this test, both on luminosity grounds and because most high-redshift QSOs will have their light blocked by a Lyman-limit absorber (galaxy or diffuse cloud) at smaller redshifts. Frantic searches turned up a few candidates, either at z > 2.8 for HST or z > 2.2 for HUT. Jakobsen et al. (1994 Nature 370, 35) reported the detection of an absorption trough in
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