The He II Opacity of the Lyman α Forest and the Intergalactic Medium

Primordial baryonic matter in the vast intergalactic space may be traced with Lyα resonance absorption by neutral hydrogen and singly ionized helium. The wavelengthaveraged He ii opacity shortward of 304(1+z) Å, as measured by low-resolution UV spectroscopy, is at least 4.5 times the H i opacity shortward of 1216(1+z) Å. While a part of the He ii opacity arises from the intergalactic regions that produce the known Lyα forest, it has been argued whether the He ii opacity may be entirely attributable to these observed lines. Based on the empirical formulas governing the distribution of Lyα forest absorption, we use a Monte-Carlo technique to calculate the average He ii optical depth produced by these forest lines. The He ii counterparts of the Lyα forest lines are highly saturated, and hence their contributions to the observed opacity is limited. Assuming a He/H0 population ratio of 100 and that the power-law distribution dn/dN ∝ N−1.5 can be extended to a neutral hydrogen column density of NHI = 2 × 10 12 cm−2, the contribution from these forest lines may account for a He ii opacity that is < ∼ 3 times the H i opacity. Our simulated He ii spectrum of the quasar Q0302-003, based on the fitted lines in the high-resolution Keck spectrum, yields a forest optical depth of ∼ 0.9, less than half of the observed He ii opacity. Therefore, a substantial contribution to He ii absorption arises from extremely tenuous regions of intergalactic gas that are beyond the observational limits for H i absorption. He ii spectra at higher resolution are a sensitive tool to explore the properties of these small-scale fluctuations that fill ∼ 80% of the intergalactic space and contain a significant part of the baryonic matter in the early universe. Subject headings: intergalactic medium; quasars –: absorption lines; cosmology