Misc,
The Cosmic Evolution of the Metallicity Distribution of Ionized Gas Traced by Lyman Limit Systems
(2016)cite arxiv:1608.02588Comment: Submitted to the ApJ. Comments welcome.
Abstract
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.
