THE COSMIC EVOLUTION OF THE METALLICITY DISTRIBUTION OF IONIZED GAS TRACED BY LYMAN LIMIT SYSTEMS

We present the first results from our KODIAQ Z survey aimed at determining the metallicity distribution and physical properties of the z greater than or similar to 2 partial and full Lyman limit systems (pLLSs and LLSs; 16.2 <= logN(H I) < 19), which are probed of the interface regions between the intergalactic medium (IGM) and galaxies. We study 31 H I-selected pLLSs and LLSs at 2.3 < z < 3.3 observed with Keck/HIRES in absorption against background QSOs. We compare the column densities of metal ions to H I and use photoionization models to assess the metallicity. The metallicity distribution of the pLLSs/LLSs at 2.3 < z < 3.3 is consistent with a unimodal distribution peaking at [X/H] similar or equal to -2. The metallicity distribution of these absorbers therefore evolves markedly with z since at z less than or similar to 1 it is bimodal with peaks at [X/H] similar or equal to -1.8 and -0.3. There is a substantial fraction (25%-41%) of pLLSs/LLSs with metallicities well below those of damped Ly alpha absorbers (DLAs) at any studied z from z less than or similar to to z similar to 2-4, implying reservoirs of metal-poor, cool, dense gas in the IGM/galaxy interface at all z. However, the gas probed by pLLSs and LLSs is rarely pristine, with a fraction of 3%-18% for pLLSs/LLSs with [X/H] <= -3. We find C/alpha enhancement in several pLLSs and LLSs in the metallicity range-2 less than or similar to [X/H] less than or similar to -0.5, where C/alpha is 2-5 times larger than observed in Galactic metal-poor stars or high-redshift DLAs at similar metallicities. This is likely caused by preferential ejection of carbon from metal-poor galaxies into their surroundings.