AAS 198th Meeting, June 2001
Session 58. The Intergalactic Medium

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[58.03] The Impact of an Early X-ray Background on the Intergalactic Medium


Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts of about 6. Here, we examine the effects of an X-ray background, such as high-energy photons from early quasars, on the temperature and ionization of the IGM before reionization has occurred, i.e., before the fully ionized bubbles associated with individual sources have overlapped. X-rays, which have a large mean free path relative to UV photons, and the photoelectrons that they liberate can have significant effects on the thermal and ionization balance. We find that hydrogen ionization is dominated by the X-ray photoionization of neutral helium and the resulting secondary electrons. Depending on the assumed high-redshift X-ray background, the IGM prior to full reionization can be heated to temperatures between a few hundred to 104 K, and ionized to levels between about 1 to 30 percent. We discuss the consequences of this for the H--catalyzed production of molecular hydrogen and the filtering of the baryonic Jeans mass scale, as well as possible signatures in the cosmic microwave background.


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