The Gunn–Peterson effect and the Lyman alpha forest

We show that spatial correlations in a stochastic large-scale velocity field in an otherwise smooth intergalactic medium (homogeneous comoving density) superposed on the general Hubble flow may cause a ‘line-like ’ structure in QSO spectra similar to the population of unsaturated Lya forest lines which usually are attributed to individual clouds with 1011 & NH i & 5 · 10 13 cm2. Therefore there is no clear observational distinction between a diffuse intergalactic medium and discrete intergalactic clouds. It follows that the H I density in the diffuse intergalactic medium might be substantially underestimated if it is determined from the observed intensity distribution near the apparent continuum in high-resolution spectra of QSOs. Our tentative estimate implies a diffuse neutral hydrogen opacity tGP, 0:3 at z, 3 and a current baryon density QIGM. 0:08, assuming a Hubble constant H0 70 km s1 Mpc1