On the interpretation of the HeII absorption in the line of sight of Q0302-003

Abstract

We point out the peculiarities of the line of sight of Q0302-003 that was recently used to estimate the HeII opacity of the Universe at z~3.285 (Jakobsen et al.). This line of sight contains a large void in Lyman alpha clouds, in the region used for the HeII opacity observation, and the void is thought to be caused by the ionizing radiation from a foreground quasar (Dobrzycki & Bechtold). If this is true, then the radiation from the foreground quasar is more relevant in the region used for the estimation of the HeII opacity than the diffuse UV background radiation. We argue that, in that case, the observed opacity should not be used to put a constraint on the diffuse UV background radiation. Using a typical spectral index of quasars, we estimate that most of the observed HeII opacity must be due to the Gunn-Peterson effect and suggest an HI Gunn-Peterson test along this line of sight. We further discuss the clumpiness of the intergalactic medium in the vicinity of Q0302-003 as suggested by observations.