1997/11/12 by Wei Zheng, A. F. Davidsen, Arthur F. Davidsen +2 · 1 citation
Physics and Astronomy · #Astrophysics and Star Formation Studies #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/300230
18 pages, including 4 figures To appear in A.J., February 1998
arxiv created 1997/11/12 · openalex publication_date 1998/02/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/01
Primordial baryonic matter in the vast intergalactic space may be traced with Lyα resonance absorption by neutral hydrogen and singly ionized helium. The wavelength-averaged He II opacity shortward of 304(1 + z ) Å, as measured by low-resolution UV spectroscopy, is at least 4.5 times greater than 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 contribution to the observed opacity is limited. Assuming an He + to H 0 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 N H I = 2 × 10 12 cm -2 , the contribution from these forest lines may account for an 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 a high-resolution Keck spectrum, yields a forest optical depth of ∼0.9, less than half 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.