Intergalactic helium absorption in cold dark matter models

Observations from the Hopkins Ultraviolet Telescope and the Hubble Space Telescope have recently detected HeII absorption along the lines of sight to two high redshift quasars. We use cosmological simulations with gas dynamics to investigate HeII absorption in the cold dark matter (CDM) theory of structure formation. We consider two Ω = 1 CDM models with different normalizations and one open universe (Ω0 = 0.4) CDM model. The simulations incorporate the photoionizing UV background spectrum computed by Haardt & Madau (1996), which is based on the output of observed quasars and reprocessing by the Lyα forest. The simulated gas distribution, combined with the Haardt & Madau spectral shape, accounts for the relative observed values of τHI and τHeII, the effective mean optical depths for HI and HeII absorption. If the background intensity is as high as Haardt & Madau predict, then matching the absolute observed values of τHI and τHeII requires a baryon abundance larger (by factors between 1.5 and 3 for the various CDM models) than our assumed value of Ωbh 2 = 0.0125. The simulations reproduce the evolution of τHeII over the observed redshift range, 2.2 ∼< z ∼< 3.3, if the HeII photoionization rate remains roughly constant. HeII absorption in the CDM simulations is produced by a diffuse, fluctuating, intergalactic medium, which also gives rise to the HI Lyα forest. Much of the HeII opacity arises in underdense regions where the HI optical depth is very low. We compute statistical properties of the HeII and HI absorption that can be used to test the CDM models and distinguish them from an alternative scenario in which the HeII absorption is caused by discrete, compact clouds. The CDM scenario predicts that a substantial amount of baryonic material resides in underdense regions at high redshift. Department of Astronomy, The Ohio State University, Columbus, OH 43210 2 Department of Astronomy, University of Washington, Seattle, WA, 98195 3 Department of Physics and Astronomy, University of Massachusetts, Amherst, MA, 98195 Lick Observatory, University of California, Santa Cruz, CA 95064 Presidential Faculty Fellow [email protected] [email protected] [email protected] [email protected]