He I absorption by Lyman-alpha clouds and low-redshift Lyman-alpha clouds

Abstract

We model the evolution of the metagalactic ionizing background radiation field, utilizing sources and sinks of radiation as in our prior work. We show that, if the comoving number density of quasars approached constancy at high redshift, as suggested by some recent observations, the observed ionizing background might possibly have been produced by the observed quasars alone. We also consider, as an extra possible source, the UV radiation from decaying neutrinos proposed by Sciama. We find that, if the background spectrum were as soft as in the Sciama scenario, the observed Lyα clouds would contain detectable amounts of neutral helium, producing a "He I forest" almost as dense as the hydrogen forest, due to the 584 A transition 1s^2^-1s2p. Current observations of three quasars, where the He I Gunn-Peterson effect (or forest decrement) might have been detected, are of too poor resolution and signal-to-noise ratio to detect the average flux decrement produced by this He I forest, predicted by Sciama's hypothesis. However, the absence of a continuous absorption trough which should be produced by a Lyman limit system detected in the spectrum of HS 1700 + 6416 rules out the existence of the decaying neutrinos, unless the He I in the Lyman limit system is ionized by a local source, such as an associated galaxy. We also note that, in the neutrino decay model, one would expect to see He I continuous absorption from several clouds in a typical line of sight, without significant continuous absorption from H i; no such clouds are observed. Thus, we believe that observations of the Lyα forest contradict the Sciama hypothesis. In most models for the ionizing background, the Lyman limit system in HS 1700+6416 should show He I absorption lines, of an equivalent width close to that of the Lyα line. The measurement of the He I column density in this system would highly constrain the shape of the spectrum of the ionizing background. This quasar is also a very good candidate to detect the He II Gunn-Peterson absorption trough or, in its absence, the He II forest. We also note that, if the ionizing background is produced by galaxies or quasars, the inevitable decline of the ionizing radiation density between a redshift of 2.5 and the present should increase the number of Lyα clouds seen at small redshift in comparison with estimates made by extrapolating the Lyα cloud number densities from higher redshifts, as has been recently observed by Morris et al. and Bahcall et al. On the other hand, if the emission of ionizing radiation is dominated by decaying neutrinos, and in the absence of other compensatory changes in the properties of the Lyα clouds, the increase in the number of low-redshift Lyα lines is not expected.