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The cosmic evolution of the metallicity distribution of ionized gas traced by Lyman limit systems.

Lehner, N. and O'Meara, J.M. and Howk, J.C. and Prochaska, J.X. and Fumagalli, M. (2016) 'The cosmic evolution of the metallicity distribution of ionized gas traced by Lyman limit systems.', Astrophysical journal., 833 (2). p. 283.


We present the first results from our KODIAQ Z survey aimed to determine the metallicity distribution and physical properties of the z>2 partial and full Lyman limit systems (pLLSs and LLSs; 16.2<log N(HI)<19), which are probed of the interface regions between the intergalactic medium (IGM) 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 HI 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 Lya 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 3-18% for pLLSs/LLSs with [X/H]<-3. We find 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 likely caused by preferential ejection of carbon from metal-poor galaxies into their surroundings.

Item Type:Article
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Publisher statement:© 2016. The American Astronomical Society. All rights reserved.
Date accepted:18 October 2016
Date deposited:03 January 2017
Date of first online publication:21 December 2016
Date first made open access:03 January 2017

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