The COS CGM Compendium. II. Metallicities of the Partial and Lyman Limit Systems at z ≲ 1

Abstract

We present the results from our COS circumgalactic medium (CGM) compendium (CCC), a survey of the CGM at z ≲ 1 using H I-selected absorbers with 15 < {log}{N}_{{H}{{I}}} < 19. We focus here on 82 partial Lyman limit systems (pLLSs, 16.2 ≤ {log}{N}_{{H}{{I}}} < 17.2) and 29 LLSs (17.2 ≤ {log}{N}_{{H}{{I}}} < 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 a 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}_{{H}{{I}}} ≥ 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.