The COS CGM Compendium. III. Metallicity and Physical Properties of the Cool Circumgalactic Medium at z ≲ 1

We characterize the metallicities and physical properties of cool, photoionized gas in a sample of 152 z less than or similar to 1 strong Ly alpha forest systems (SLFSs, absorbers with 15 < log N-H I < 16.2). The sample is drawn from our Cosmic Origins Spectrograph (COS) circumgalactic medium compendium (CCC), an ultraviolet survey of H I-selected circumgalactic gas around z less than or similar to 1 galaxies that targets 261 absorbers with 15 < log N-H I < 19. We show that the metallicity probability distribution function of the SLFSs at z less than or similar to 1 is unimodal, skewed to low metallicities with a mean and median of [X/H] = -1.47 and -1.18 dex. Very metal-poor gas with [X/H] < -1.4 represents about half of the population of absorbers with 15 < log N-H I less than or similar to 18, while it is rare at higher N-H I. Thus, there are important reservoirs of primitive (though not pristine) diffuse ionized gas around z less than or similar to 1 galaxies. The photoionized gas around z less than or similar to 1 galaxies is highly inhomogeneous based on the wide range of metallicities observed (-3 less than or similar to [X/H] less than or similar to +0.4) and the fact that there are large metallicity variations (factors of 2 to 25) for most of the closely spaced absorbers (Delta(v) less than or similar to 300 kms(-1)) along the same sightlines. These absorbers show a complex evolution with redshift and H I column density, and we identify subtle cosmic evolution effects that affect the interpretation of metallicity distributions and comparison with other absorber samples. We discuss the physical conditions and cosmic baryon and metal budgets of the CCC absorbers. Finally, we compare the CCC results to recent cosmological zoom simulations and explore the origins of the 15 < log N-H I < 19 absorbers within the Evolution and Assembly of GaLaxies and their Environments (EAGLE) high-resolution simulations.