On the Origin of Metallicity in Lyα Forest Systems

We investigate the hypothesis that $\lyal$ absorption lines arise in twopopulations of halos --- minihalos of small circular velocity ($V_c \la 55$ kms$^{-1}$) in which star formation and metal production are inhibited byphotoionization 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 themodel of $\lyal$-absorbing gas confined in both populations of halos, weattempt to explain the recent observations of (1) associations of visiblegalaxies with $\lyal$ lines at low redshifts $z \la 1$, and (2) metal linesassociated 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 cloudsconfined in the pressure of ambient hot gas can produce $\lyal$ absorptionswith H I column density as low as $10^{14}$ cm$^{-2}$, and that the impactparameter of a sightline for such absorptions matches well with the observedradius of gaseous envelope in a typical luminous galaxy. Using the Press-Schechter prescription for the mass function of halos, we also show that thefraction of $\lyal$ lines with associated metal lines can be understood interms of the fraction of $\lyal$ absorbers that are associated with galactichalos. 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 galactichalos is $40 \sim 60$ km s$^{-1}$, which is consistent with the theoreticalprediction of galaxy formation under photoionization. Comment: 22 pages, LaTex (aaspp4.sty), 6 postscript figures, To appear in Ap