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 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.
