Chemical Enrichment at High Redshifts

We have tried to understand the recent observations related to metallicity in Lyα forest clouds in the framework of the two-component model suggested by Chiba & Nath. The model consists of mini-halos having circular velocities smaller than ~55 km s^-1 with no star formation and galactic halos with higher circular velocities <=250 km s^-1 having clouds, star formation, and consequent metal enrichment. The number of clouds in the galactic halos was determined from the observed redshift distribution of Lyα lines. We find that even if the mini-halos were chemically enriched by an earlier generation of stars, to have [C/H]~=-2.5, the number of C IV lines with column density >10¹² cm^-2, contributed by themini-halos at the redshift of 3, would be only about 10% of the total number of lines for a chemical enrichment rate of (1+z)^-3 in the galaxies. Not much information about the degree of enrichment of the mini-halos can therefore be obtained from observations of C IV lines with column density >10¹² cm^-2. The recently reported absence of heavy-element lines associated with most of the Lyα lines withH I column density between 10^13.5 and 10¹⁴ cm^-2 by Lu and coworkers, if correct, gives an upper limit on [C/H]=-3.7 not only in the mini-halos, but also in the outer parts of galactic halos. This is consistent with the results of numerical simulations, according to which the chemical elements associated with the Lyα clouds are formed in situ in clouds rather than in an earlier generation of stars. However, the mean value of 7×10^-3 for the column density ratio of C IV and H I, determined by Cowie & Songaila for low-Lyα optical depths, implies an abundance of [C/H]=-2.5 in mini-halos as well as most of the region in galactic halos, presumably enriched by an earlier generation of stars. The redshift and column density distribution of C IV has been shown to be in reasonable agreement with the observations.