Chemical history at z ^ 2.0: first results from the Magellan + Keck survey of Lyman limit systems

摘要

We present a measurement of [O/Fe] versus [Fe/H] in the z > 2 IGM derived fiom a sample of 23 Lyman Limit systems whose column density lies in the range 19.0 ^log JV/f/^ 20 2 cm"2. The systems span a metallicity range of -2,70 ^ [Fe/H] ^ -0,50, allowing for a direct comparison with similar studies of stars within the Milky Way, Although the Lyman Limit systems can be highly ionised, the effects of ionisation on the determination of [O/Fe] is minor for most of the systems in the sample. The data appear to show a decrease in [O/Fe] with increasing metallicity with an approximate slope of m ~ -0.55. We also determine a best fit power law slope of a = -1 86±0 2 to the column density distribution fni(N) for 19 0 ^ log Ngj ^ 20 2 cm"2.