Chemical enrichment at high redshifts

Abstract

We have tried to understand the recent observations related to metallicity in Ly-α forest clouds in the framework of the two component model. 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. 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 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)⁻³ in the galaxies. Recently reported absence of heavy element lines associated with most of the Ly-α lines with HI column density between 10^13.5 cm⁻² and 10¹⁴ cm⁻² by Lu et al [13], 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. However, the mean value of 7×10⁻³ for the column density ratio of C IV and HI, determined by Cowie and Songaila (1998) for low Ly-α optical depths, implies an abundance of [C/H]= −2.5 in mini-halos as well as most of the region in galactic halos. The redshift and column density distribution of C IV has been shown to be in reasonable agreement with the observations.