On the ionization balance of helium in the intergalactic medium

A straightforward photoionization analysis is given of the tentative IUE observation that the intergalactic medium near the QSO Q2204 – 408 (z = 3 .18) contains a He II abundance of at least $$2\,\times\,{10}^{-9}\,\text{cm}^{-3}$$. This analysis is found to be rather implausible and it is suggested that, if the IGM is indeed photoionized, the QSO may be located in an expanding supercluster of size ≳ 13 Mpc and total mean gas density $$\sim\,1.5\,\times\,{10}^{-5}\,\text{cm}^{-3}$$. If the same is true for OQ 172 (z = 3.53), one might be able to use IUE to detect the edge of its supercluster in He II near the plate limit.