The COS CGM Compendium. II: Metallicities of the Partial and Lyman Limit
  Systems at z<1
  We present the results from our COS circumgalactic medium (CGM) compendium
(CCC), a survey of the CGM at z<1 using HI-selected absorbers with 15<log N(HI)
<19. We focus here on 82 partial Lyman limit systems (pLLSs, 16.2<log N(HI)
<17.2) and 29 LLSs (17.2<log N(HI) <19). Using Bayesian techniques and
Markov-chain Monte Carlo sampling of a grid of photoionization models, we
derive the posterior probability distribution functions (PDFs) for the
metallicity of each absorber in CCC. We show that the combined pLLS metallicity
PDF at z<1 has two main peaks at [X/H]=-1.7 and -0.4, with a strong dip at
[X/H]=-1. The metallicity PDF of the LLSs might be more complicated than an
unimodal or bimodal distribution. The pLLSs and LLSs probe a similar range of
metallicities -3<[X/H]<+0.4, but the fraction of very metal-poor absorbers with
[X/H]<-1.4 is much larger for the pLLSs than the LLSs. In contrast, absorbers
with log N(HI)>19 have mostly -1<[X/H]<0 at z<1. The metal-enriched gas probed
by pLLSs and LLSs confirms that galaxies that have been enriching their CGM
over billions of years. Surprisingly, despite this enrichment, there is also
abundant metal-poor CGM gas (41-59% of the pLLSs have [X/H]<-1.4), representing
a reservoir of near-pristine gas around z<1 galaxies. We compare our empirical
results to recent cosmological zoom simulations, finding some discrepancies,
including an overabundance of metal-enriched CGM gas in simulations.
