Chemical enrichment at high redshifts
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 1013.5 cm−2 and 1014 cm−2 by Lu et al , 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.
Volume 96, 2022
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