Helium Absorption by Protogalactic Gas

We present a new, high resolution HST/GHRS spectrum of quasar Q0302-003, and use the ${\rm He^+}$ Lyman-$\alpha$ absorption, together with a high resolution Keck spectrum of the HI Lyman-$\alpha$ forest, to probe the distribution and ionization state of the protogalactic foreground gas at $z\approx 3.3$. Within $\approx 4000$ km/sec of the quasar redshift the spectrum shows a substantial flux ($\tau \approx 1$) with``${\rm He^+}$ Lyman-$\alpha$ forest'' absorption features correlated in redshift with the HI Lyman-$\alpha$ forest; the absorption in this region (including the main ``Gunn-Peterson edge'') is accounted for entirely by the discrete components of the forest. We attribute the lack of continuous absorption from diffuse gas to the ``proximity effect'', a large bubble doubly ionized by the quasar, and use its size to estimate the background flux at the ${\rm He^+}$ ionization threshold; the lack of continuous absorption also leads to a limit on diffuse gas density near the quasar, tied to the quasar flux. Far from the quasar the spectrum displays ${\rm He^+}$ absorption ($\tau\ge 1.3$) even in redshift intervals with no detectable HI absorption, implying some smoothly distributed gas ($\Omega_g\ge 3\times 10^{-5}$), a ${\rm He^+}$ to HI ratio $\eta\ge 100$, and a soft ionizing background. The spectrum also displays residual nonzero flux everywhere (average $\tau\approx 2$), which indicates a low diffuse gas density $\Omega_g\approx 0.01$.