Did Cosmic Rays Reionize the Intergalactic Medium?

We investigate the role of cosmic rays from young galaxies in heating and ionizing the intergalactic medium (IGM) at high redshift. Using the IRAS observations at $60 μm$, we estimate the cosmic ray luminosity density at the present epoch. We consider various forms of luminosity evolution in redshift and calculate (a) the thresholds corresponding to the upper limits of Gunn-Peterson optical depth, (b) the Compton $y$ parameter for an IGM heated by cosmic rays and compare with the upper limits from COBE measurements and (c) an estimated limit from the integral of metal enrichment. We show that certain models, with rather strong evolution and early formation of galaxies, allow reionization of the IGM, consistent with all known constraints.

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