The Full Re-Ionization of Helium

Observations of resolved HeII Lyman alpha absorption in spectra of two QSO’s suggest that the epoch of helium ionization occurred at z ≈ 3. Proximity zones in the spectra of the quasars (z = 3.18, 3.285) at 304 Å resemble Stromgren spheres, suggesting that the intergalactic medium is only singly ionized in helium. We present models of the proximity effect which include the full physics of the ionization, heating and cooling and an accurately simulated inhomogeneous gas distribution. In these models the underdense intergalactic medium is heated to at least 10,000-20,000 K after cooling to as low as a few 1000K due to cosmological expansion, with higher temperatures achieved farther away from the quasar due to absorption-hardened ionizing spectra. The quasars turn on for a few ×107 years with a fairly steady flux output at 228 Å comparable to the 304 Å flux output directly observed with HST. The recoveries in the spectra occur naturally due to voids in the IGM and may provide a fairly model-independent probe of the baryon density. In the last few years it has become possible to observe details of absorption by singly ionized helium. The observations combine new information about the history of quasars, intergalactic gas, and structure formation. These phenomena can be disentangled with detailed quantitative models of the situation which we briefly describe here. Theoretical treatments of some of the effects modeled here were given by Zheng and Davidsen (1995), Croft et al. (1997), Miralda-Escudé et al. (1996), Zhang et al. (1998) and Fardal, Giroux and Shull (1998). Early observations of the helium II Lyman alpha absorption spectral region included the quasars Q0302-003 (z=3.285, Jakobsen et al. 1994), HS 1700+64 (z=2.72, Davidsen et al. 1996) and PKS 1935-692 (z=3.18, Tytler & Jakobsen 1996). Higher resolution (GHRS) observations of Q0302-003 (Hogan, Anderson & Rugers 1997) and HE 2347-4342 (z=2.885, Reimers et al. 1997) revealed structure in the absorption which could be reliably correlated with HI absorption. The most recent published observations of PKS 1935-692 with STIS PKS 1935-692: STIS & GHRS HeII, CTIO HI spectra 123