Intergalactic Helium Absorption in Cold Dark Matter Models

Observations from the Hopkins Ultraviolet Telescope and the Hubble Space Telescope have recently detected He II absorption along the lines of sight to two high-redshift quasars. We use cosmological simulations with gas dynamics to investigate He II absorption in the cold dark matter (CDM) theory of structure formation. We consider two ) \ 1 CDM models with di erent normalizations and one open universe CDM model. The simulations incorporate the photoionizing UV background spec-() 0 \ 0.4) trum computed by Haardt & Madau, which is based on the output of observed quasars and reprocessing by the Lya forest. The simulated gas distribution, combined with the Haardt & Madau spectral shape, accounts for the relative observed values of and the e ective mean optical depths for H I q6 H I q6 He II , and He II absorption. If the background intensity is as high as Haardt & Madau predict, then matching the absolute observed values of and requires a baryon abundance larger (by factors between 1.5 q6 H I q6 He II and 3 for the various CDM models) than our assumed value of The simulations repro-) b h2 \ 0.0125. duce the evolution of over the observed redshift range, if the He II photoionization q6 He II 2.2 [ z [ 3.3, rate remains roughly constant.