The Cosmic Evolution of the Metallicity Distribution of Ionized Gas Traced by Lyman Limit Systems

Nicolas Lehner, John M. O'Meara, J. Christopher Howk, J. Xavier Prochaska, Michele Fumagalli

Published 2016-08-08Version 1

We present the first results from our KODIAQ Z survey aimed to determine the metallicity distribution and physical properties of the partial and full Lyman limit systems (pLLSs and LLSs; 16.2<log N(HI)<19) at z>2, which probe gas in the interface regions between the intergalactic medium and galaxies. We study 31 HI-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 N(HI) and use photoionization models to assess the total H column density and the metallicity. The metallicity distribution function (MDF) of the pLLSs/LLSs at 2.3<z<3.3 is consistent with a unimodal distribution peaking at [X/H]=-2, in contrast to the bimodal MDF seen at z<1. There is a substantial fraction (20-45%) of pLLSs/LLSs with metallicities well below those of damped Lyman alpha absorbers (DLAs) at any given z, and this fraction remains relatively constant from z<1 to z~2-4. There is therefore a reservoir of metal-poor cool gas at all z that may eventually accrete onto galaxies. We find that on average the metallicity increases with increasing N(HI) from the Lyman alpha forest, to the pLLSs/LLSs, to the damped Lyman alpha absorbers (DLAs). Finally there is evidence for C/alpha enhancement in several pLLSs and LLSs in the metallicity range -2<[X/H]<-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 possibly caused by preferential ejection of carbon from metal-poor galaxies into their surroundings.