A POPULATION OF VERY DIFFUSE LYMAN-ALPHA CLOUDS AS THE ORIGIN OF THE HE+ ABSORPTION SIGNAL IN THE INTERGALACTIC MEDIUM

KNOWLEDGE of the physical state of the relatively uniform component of the intergalactic medium (the 'substrate') is critical to understanding the propagation of ionizing radiation and dynamical energy through intergalactic space, and for establishing the boundary conditions for the formation of the intergalactic gas clouds and galaxies that are assumed to have condensed from it. Uniformly distributed hydrogen, and, even more so, He+ will produce characteristic smooth absorption in the spectra of high-redshift quasars(1-4), but at low spectral resolution it is difficult to distinguish such an absorption trough from the cumulative effect of absorption by the Lyman-alpha 'forest' of clouds, We report the detection of a population of weak 'forest' clouds with column density down to 2 x 10(12) cm(-2), and show that absorption in these clouds can account for a recent measurement(1) of strong He+ absorption without necessarily having to invoke a diffuse intergalactic medium.