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

We present a measurement of [{O}/{Fe}] versus [{Fe}/{H}] in the z>2 igm derived from a sample of 23 Lyman limit systems whose column density lies in the range 19.0≤log N_HI≤ 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 α=-1.86 ± 0.2 to the column density distribution f_HI(N) for 19.0 ≤log N_HI≤ 20.2 cm^-2.