CHEMICAL ENRICHMENT AT HIGH REDSHIFTS SNIGDHA DAS and PUSHPA KHARE

We have tried to understand the recent observations related to metallicity in Ly α forest clouds in the framework of the two component model suggested by Chiba & Nath (1997). The model consists of mini-halos having circular velocities smaller than ∼ 55 km s, with no star formation and galactic halos with higher circular velocities ≤ 250 km s, having clouds, star formation and consequent metal enrichment. The number of clouds in the galactic halos was determined from the observed redshift distribution of Ly α 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 >10 cm, contributed by the mini-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) 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 >10 cm. Recently reported absence of heavy element lines associated with most of the Ly α lines with H I column density between 10 cm and 10 cm by Lu et al (1998), 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 Ly α clouds are formed in situ in clouds, rather than in an earlier generation of stars. However, the mean value of 7×10 for the column density ratio of C IV and H I, determined by Cowie and Songaila (1998) for low Lyman alpha 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. Subject headings: galaxies: abundances – galaxies: halos – quasars: absorption lines