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 ≳ 2 partial and full Lyman limit systems (pLLSs and LLSs; 16.2 ≪ logNH I > 19), which are probed of the interface regions between the intergalactic medium (IGM) and galaxies. We study 31 H Iselected 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] ≲ -2. The metallicity distribution of these absorbers therefore evolves markedly with z since at z ≲ 1 it is bimodal with peaks at [X H] ≲ -1.8 and -0.3. There is a substantial fraction (25%-41%) of pLLSs/LLSs with metallicities well below those of damped Ly? absorbers (DLAs) at any studied z from z ≲ 1 to z ∼ 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/α enhancement in several pLLSs and LLSs in the metallicity range -2 ≲ [X H] ? -0.5, where C/α 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.