Mapping Low-Density Intergalactic Gas: A Third Helium LYalpha Forest

Abstract

We present a new HST/STIS spectrum of the z=3.18 quasar PKS 1935-692 and summarize the spectral features shortward of 304 Å in the rest frame likely to be caused by foreground He II Lyalpha absorption. In accord with the previous results on two other quasars at similar redshifts, we demonstrate a correlation with the H I Lyalpha forest absorption and show that much of the helium absorption is caused by a comparable quantity of more diffuse gas with Omega_g~10^-2, which is not detected in H I. The helium ionization zone around the quasar is detected, and a void is seen in both H I and He II. 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.