THE HE-II OPACITY OF THE LY-ALPHA FOREST AND THE INTERGALACTIC MEDIUM

oe Primordial baryonic matter in the vast intergalactic space may be t raced with Ly alpha resonance absorption by neutral hydrogen and singl y ionized helium. The wavelength-averaged He II opacity shortward of 3 04(1 + z) Angstrom as measured by low-resolution UV spectroscopy, is a t least 4.5 times greater than the H I opacity shortward of 1216(1 + z ) Angstrom. While a part of the He II opacity arises from the intergal actic regions that produce the known Ly alpha forest, it has been argu ed whether the He II opacity may be entirely attributable to these obs erved lines. Based on the empirical formulas governing the distributio n of Ly alpha forest absorption, we use a Monte Carlo technique to cal culate the average He II optical depth produced by these forest lines. The He II counterparts of the Ly alpha forest lines are highly satura ted, 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 proportional to N-1.5 can be extended to a neutral hydrogen column density of N-HI = 2 x 10(12)cm(-2), the contribution f rom these forest lines may account for an He II opacity that is less t han or similar to 3 times the H I opacity. Our simulated He II spectru m of the quasar Q0302-003, based on the fitted lines in a high-resolut ion Keck spectrum, yields a forest optical depth of similar to 0.9, le ss than half the observed He II opacity. Therefore, a substantial cont ribution to He II absorption arises from extremely tenuous regions of intergalactic gas that are beyond the observational limits for H I abs orption. He II spectra at higher resolution are a sensitive tool to ex plore the properties of these small-scale fluctuations that fill simil ar to 80% of the intergalactic space and contain a significant part of the baryonic matter in the early universe.