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

Abstract: We characterize the metallicities and physical properties of cool, photoionized gas in a sample of 152 z<1 strong Lya forest systems (SLFSs, absorbers with 15<log N(HI)<16.2). The sample is drawn from our COS circumgalactic medium (CGM) compendium (CCC), an ultraviolet survey of HI-selected circumgalactic gas around z<1 galaxies that targets 262 absorbers with 15<log N(HI)<19. We show that the metallicity probability distribution function of the SLFSs at z<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(HI)<18. Thus, there are important reservoirs of primitive (though not pristine) gas around z<1 galaxies. The photoionized gas around z<1 galaxies is highly inhomogeneous based on the wide range of metallicities observed (-3<[X/H]<+0.4) and that there are large metallicity variations (factors of 2 to 25) for most of the closely-spaced absorbers (Dv<300 km/s) along the same sightlines. These absorbers show a complex evolution with redshift and HI column density, and we identify subtle cosmic evolution effects that affect the interpretation of metallicity distributions and comparison with other of absorbers 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(HI)<19 absorbers within the EAGLE high-resolution simulations.