On the origin of metallicity in ly alpha forest systems

Abstract

We investigate the hypothesis that Ly alpha absorption lines arise in two populations of halos---minihalos of small circular velocity (Vc <~ 55 km s-1), in which star formation and metal production are inhibited by photoionization of the UV background radiation, and large galactic halos (55 <~ Vc <~ 250 km s-1) that contain stars and metals. Based on the model of Ly alpha -absorbing gas confined in both populations of halos, we attempt to explain the recent observations of (1) associations of visible galaxies with Ly alpha lines at low redshifts z <~ 1 and (2) metal lines associated with a nonnegligible fraction of low H I column density Ly alpha lines at z ~ 3. For galactic halos, we find that photoionized gas clouds confined in the pressure of ambient hot gas can produce Ly alpha absorption for H I column densities as low as 1014 cm-2 and that the impact parameter of a sight line for such absorptions matches well with the observed radius of the gaseous envelope of a typical luminous galaxy. Using the Press-Schechter prescription for the mass function of halos, we also show that the fraction of Ly alpha lines with associated metal lines can be understood in terms of the fraction of Ly alpha absorbers associated with galactic halos. In particular, the reported fraction of ~0.5--0.75 at z ~ 3 is reproduced when the boundary value of Vc to separate minihalos or galactic halos is 40--60 km s-1, which is consistent with the theoretical prediction of galaxy formation under photoionization. The average metallicity of both Ly alpha forest and damped Ly alpha systems at z ~ 3 is explained in terms of the model of halo-formation history combined with the age-metallicity relationship of Galactic halo stars. Possible methods to test this hypothesis and alternative scenarios are also discussed.