Helium Absorption by Protogalactic Gas

We present a new, high resolution HST/GHRS spectrum of quasar Q0302003, and use the He Lyman-α absorption, together with a high resolution Keck spectrum of the HI Lyman-α forest, to probe the distribution and ionization state of the protogalactic foreground gas at z ≈ 3.3. Within ≈ 4000 km/sec of the quasar redshift the spectrum shows a substantial flux (τ ≈ 1) with “He Lyman-α forest” absorption features correlated in redshift with the HI Lyman-α 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 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 He absorption (τ ≥ 1.3) even in redshift intervals with no detectable HI absorption, implying some smoothly distributed gas (Ωg ≥ 3 × 10 ), a He to HI ratio η ≥ 100, and a soft ionizing background. The spectrum also displays residual overall flux everywhere (average τ ≈ 2), which indicates a low diffuse gas density Ωg ≈ 0.01.