On The Origin Of Metallicity In Lyman-Alpha Forest Systems

We investigate the hypothesis that $\lyal$ absorption lines arise in two populations of halos --- minihalos of small circular velocity ($V_c \la 55$ km s$^{-1}$) in which star formation and metal production are inhibited by photoionization of the UV background radiation, and large galactic halos ($55 \la V_c \la 250$ km s$^{-1}$) which possess stars and metals. Based on the model of $\lyal$-absorbing gas confined in both populations of halos, we attempt to explain the recent observations of (1) associations of visible galaxies with $\lyal$ lines at low redshifts $z \la 1$, and (2) metal lines associated with a non-negligible fraction of low H I column density $\lyal$ lines at $z \sim 3$. For galactic halos, we find that photoionized gas clouds confined in the pressure of ambient hot gas can produce $\lyal$ absorptions with H I column density as low as $10^{14}$ cm$^{-2}$, and that the impact parameter of a sightline for such absorptions matches well with the observed radius of gaseous envelope in a typical luminous galaxy. Using the Press- Schechter prescription for the mass function of halos, we also show that the fraction of $\lyal$ lines with associated metal lines can be understood in terms of the fraction of $\lyal$ absorbers that are associated with galactic halos. In particular, the reported fraction of $\sim 0.5--0.75$ at $z \sim 3$ is reproduced when the boundary value of $V_c$ to separate mini or galactic halos is $40 \sim 60$ km s$^{-1}$, which is consistent with the theoretical prediction of galaxy formation under photoionization. .....