Intergalactic Helium Absorption in Cold Dark Matter Models

Observations from the HUT and the HST have recently detected HeII absorptionalong the lines of sight to two high redshift quasars. We use cosmologicalsimulations with gas dynamics to investigate HeII absorption in the cold darkmatter (CDM) theory of structure formation. We consider two Omega=1 CDM modelswith different normalizations and one Omega_0=0.4 CDM model, all incorporatingthe photoionizing UV background spectrum computed by Haardt & Madau (1996). Thesimulated gas distribution, combined with the H&M spectral shape, accounts forthe relative observed values of taubar_HI and taubar_HeII, the effective meanoptical depths for HI and HeII absorption. If the background intensity is ashigh as H&M predict, then matching the absolute values of taubar_HI andtaubar_HeII requires a baryon abundance larger (by factors between 1.5 and 3for the various CDM models) than our assumed value of Omega_b h^2=0.0125. Thesimulations reproduce the evolution of taubar_heII over the observed redshiftrange, 2.2 < z < 3.3, if the HeII photoionization rate remains roughlyconstant. HeII absorption in the CDM simulations is produced by a diffuse,fluctuating, intergalactic medium, which also gives rise to the HI ly-alphaforest. Much of the HeII opacity arises in underdense regions where the HIoptical depth is very low. We compute statistical properties of the HeII and HIabsorption that can be used to test the CDM models and distinguish them from analternative scenario in which the HeII absorption is caused by discrete,compact clouds. The CDM scenario predicts that a substantial amount of baryonicmaterial resides in underdense regions at high redshift. HeII absorption is theonly sensitive probe of such extremely diffuse, intergalactic gas, so it canprovide a vital test of this fundamental prediction.