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We present a new, high resolution HST/GHRS spectrum of quasar Q0302-003, and use the He (+) Lyman-alpha absorption, together with a high resolution Keck spectrum of the HI Lyman-alpha forest, to probe the distribution and ionization state of foreground gas just below the quasar redshift z~ 3. 3. Within ~ 4000 km/sec of the quasar redshift the spectrum shows a substantial flux (tau ~ 1) with ``He (+) Lyman-alpha forest'' absorption features correlated in redshift with the HI Lyman-alpha forest; the absorption in this region is accounted for entirely by the discrete components of the forest, indeed the main ``Gunn-Peterson edge'' can be identified with a particular complex of HI absorbing clouds. We attribute the lack of continuous absorption from diffuse gas to the ``proximity effect'' in this region, a large bubble where helium is highly ionized by the quasar, and use its size to estimate the background flux at the He (+) ionization threshold. The near-quasar data also lead to constraints on diffuse gas density near the quasar, tied to the observed quasar flux, and helium abundance, tied to the observed quasar spectrum. Far from the quasar redshift, the spectrum displays He (+) absorption (tau >= 1. 3) even in several redshift intervals with no detectable HI absorption, implying a soft ionizing spectrum as well as absorption from gas between detected HI clouds. The smoothed spectrum displays residual flux everywhere with an average optical depth tau_ GP<=2, which indicates a low density of redshift-space-filling gas; using constraints from the HI ionizing spectrum we estimate Omegag <= 0. 01 (h/0. 7) (-1. 5), and infer that the helium is already mostly doubly ionized by this epoch. Our estimates are consistent with ionization models based on observed quasar populations, previous limits from HI Gunn-Peterson studies, and simulations of the gas distribution in CDM models of galaxy formation.
Hogan et al. (Thu,) studied this question.