ON THE ORIGIN OF METALLICITY IN LY-ALPHA FOREST SYSTEMS

We investigate the hypothesis that Ly alpha absorption lines arise in two populations of halos-minihalos of small circular velocity (V-c les s than or similar to 55 km s(-1)), in which star formation and metal p roduction are inhibited by photoionization of the UV background radiat ion, and large galactic halos (55 less than or similar to V-c less tha n or similar to 250 km s(-1)) that contain stars and metals. Based on the model of Ly alpha-absorbing gas confined in both populations of ha los, we attempt to explain the recent observations of(1) associations of visible galaxies with Ly alpha lines at low redshifts z less than o r similar to 1 and (2) metal lines associated with a nonnegligible fra ction of low H I column density Ly alpha lines at z similar to 3. For galactic halos, we find that photoionized gas clouds confined in the p ressure of ambient hot gas can produce Ly alpha absorption for H I col umn densities as low as 10(14) cm(-2) and that the impact parameter of a sight line for such absorptions matches well with the observed radi us of the gaseous envelope of a typical luminous galaxy. Using the Pre ss-Schechter prescription for the mass function of halos, we also show that the fraction of Ly alpha lines with associated metal lines can b e understood in terms of the fraction of Ly alpha absorbers associated with galactic halos. In particular, the reported fraction of similar to 0.5-0.75 at z similar to 3 is reproduced when the boundary value of V-c, to separate minihalos or galactic halos is 40-60 km s(-1), which is consistent with the theoretical prediction of galaxy formation und er photoionization. The average metallicity of both Ly alpha forest an d damped Ly alpha systems at z similar to 3 is explained in terms of t he model of halo-formation history combined with the age-metallicity r elationship of Galactic halo stars. Possible methods to test this hypo thesis and alternative scenarios are also discussed.