On the source of ionization of the intergalactic medium at z~2.4

Abstract

We use the recent detection of H ii absorption at z=2.2-2.6 in the line of sight of the quasar HS 1700+64 to put bounds on the sources of ionization. We find that, given the uncertainty in tau^HI_GP and the model of absorption in the intergalactic medium (IGM), a wide range of 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 photoionization by the UV background radiation, the contribution of star-forming galaxies to the intensity at 912 A can be within a range of ~1-15 times that of the quasars. We also investigate the case of a collisionally ionized IGM. We show that although collisional ionization can be a dominant source of ionization at z~2.4 (taking into account the upper limit on the Compton y-parameter of the microwave background radiation), the thermal state of the IGM cannot 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.