1998A&A...331...61J

Intervening intergalactic HeII λ304 absorption has so far been detected toward four high redshift quasars by HST and HUT. The quantitative interpretation of these data provides a measure of the ionization in the absorbing gas in the form of the ratio of residual HeII ions to HI atoms. Assuming a standard Big Bang helium abundance, this ratio can be used to derive a lower limit on the baryonic content of the Lyman forest without reference to the intensity of the ionizing background flux. It is shown that the HeII detections are consistent with the ionized component of the Lyman forest being a dominant carrier of baryonic matter at high redshift. If the detected HeII absorption is attributed exclusively to the known population of Lyman forest clouds with HI column densities N_HI>10¹²cm^–2, then lower limits for the cosmological density of the Lyman forest of {OMEGA}_bh₇₅>1x10^–2 at <z≥∼2.4, and {OMEGA}_bh₇₅>9x10^–2 at <z≥∼3.0, are implied by the HUT and HST data, respectively. Both these lower limits are uncomfortably large compared to recent determinations of {OMEGA}_ b_ based on the D/H ratio in the forest clouds, but can be lowered by adding a contribution from more diffuse gas, either in the form of very low column density forest clouds or a uniform intergalactic medium. While good consistency between the HeII and D/H values of {OMEGA}_b can be reached in the <z≥∼2.4 case in this way, the very strong HeII absorption seen at <z≥∼3.0 is only barely reconcilable with the D/H data. This suggests that the underlying assumption of photoionization equilibrium may not apply to HeII at this redshift, thereby providing further indirect evidence for HeII reionization occurring at z≃2.9.