We use ROSAT/ASCA X-ray spectra of high-redshift AGNs to study the intergalactic medium in the deep Universe. Because the primordial gas is likely completely ionized but not the intergalactic metals, some soft X-ray absorption features are therefore expected to be detected in the spectra of distant AGNs. We have calculated the ionization state of a metal-rich IGM, and applied the model to the joint ASCA-ROSAT X-ray spectral analysis of four AGNs at redshifts ranging from 1.8 and up to nearly 5. This test provides a constraint on the temperature and density of the IGM, which gives a well consistent picture of the physical properties of the IGM, in comparison with the results obtained from the traditional Gunn-Peterson test, and constraints given by both the cosmic microwave background and the X-ray background.
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