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 Iselected 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 metalpoor 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 metalenriched 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 metalpoor CGM gas (41%59% of the pLLSs have [{{X}}/{{H}}] ≲ 1.4), representing a reservoir of nearpristine gas around z ≲ 1 galaxies. We compare our empirical results to recent cosmological zoom simulations, finding some discrepancies, including an overabundance of metalenriched CGM gas in simulations.
 Publication:

The Astrophysical Journal
 Pub Date:
 February 2019
 DOI:
 10.3847/15384357/aafb74
 arXiv:
 arXiv:1811.10654
 Bibcode:
 2019ApJ...872...81W
 Keywords:

 cosmology: observations;
 galaxies: abundances;
 galaxies: evolution;
 galaxies: halos;
 intergalactic medium;
 quasars: absorption lines;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 Resubmitted to ApJ after the first referee report