Mapping Low-Density Intergalactic Gas: a Third Helium Lyman-α Forest

We present a new HST/STIS spectrum of the z = 3.18 quasar PKS 1935-692 and summarize the spectral features shortwards of 304 Å in the rest frame likely to be caused by foreground HeII Lyman-α absorption. In accord with previous results on two other quasars at similar redshifts, we demonstrate a correlation with the HI Lyman-α forest absorption, and show that much of the helium absorption is caused by a comparable quantity of more diffuse gas with Ωg ∼ 10, that is not detected in HI. The helium ionization zone around the quasar is detected as well as a void seen in both HI and HeII. The properties of the absorption are in broad agreement with those of the other quasars and with models of the protogalactic gas distribution and ionization at this redshift. Subject headings: cosmology: observations — intergalactic medium — quasars: absorption lines — quasars: individual (PKS 1935-692)