ON THE SOURCE OF IONIZATION OF THE INTERGALACTIC MEDIUM AT Z-SIMILAR-TO-2.4

We use the recent detection of H II absorption at z=2.2-2.6 in the lin e of sight of the quasar HS 1700 + 64 to put bounds on the sources of ionization. We find that, given the uncertainty in tau(GP)(HI) and the model of absorption in the intergalactic medium (IGM), a wide range o f possible sources of ionization is still allowed by the observations. We show that a significant contribution from star-forming galaxies is allowed and is consistent with the proximity effect. In the case of p hotoionization by the UV background radiation, the contribution of sta r-forming galaxies to the intensity at 912 Angstrom can be within a ra nge of similar to 1-15 times that of the quasars. We also investigate the case of a collisionally ionized IGM. We show that although collisi onal ionization can be a dominant source of ionization at z similar to 2.4 (taking into account the upper limit on the Compton y-parameter o f the microwave background radiation), the thermal state of the IGM ca nnot yet be determined. We also consider Sciama's model of radiatively decaying neutrinos, and show that the model of decaying neutrinos is consistent with the observations.