Chemical enrichment at high redshifts

We have tried to understand the recent observations related to metallicity in Ly alpha forest clouds in the framework of the two-component modal sugge sted by Chiba & Nath. The model consists of mini-halos having circular velo cities smaller than similar to 55 km s(-1) with no star formation and galac tic halos with higher circular velocities less than or equal to 250 km s(-1 ) having clouds, star formation, and consequent metal enrichment. The numbe r of clouds in the galactic halos was determined from the observed redshift distribution of Ly alpha lines. We find that even if the mini-halos were c hemically enriched by an earlier generation of stars, to have [C/H] similar or equal to -2.5, the number of C Iv lines with column density > 10(12) cm (-2), contributed by the mini-halos at the redshift of 3, would be only abo ut 10% of the total number of lines for a chemical enrichment rate of (1 z)(-3) in, th, galaxies. Not much information about the degree of enrichmen t of the mini-halos can therefore be obtained from observations of C Iv lin es with column density >10(12) cm(-2). The recently reported absence of hea vy-element lines associated with most of the Ly alpha lines with H I column density between 10(13.5) and 10(14) 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 associat ed with the Ly alpha clouds are formed in situ in clouds rather than in an earlier generation of stars. However, the mean value of 7 x 10(-3) for the column density ratio of C Iv and H I, determined by Cowie & Songaila for ow -Ly alpha optical depths, implies an abundance of [C/H] = -2.5 in mini-halo s as well as most of the region in galactic halos, presumably enriched by a n earlier generation of stars. The redshift and column density distribution of C Iv has been shown to be in reasonable agreement with the observations .