2021MNRAS.506..877Z

We present a systematic investigation of physical conditions and elemental abundances in four optically thick Lyman-limit systems (LLSs) at z = 0.36-0.6 discovered within the cosmic ultraviolet baryon survey (CUBS). Because intervening LLSs at z < 1 suppress far-UV (ultraviolet) light from background QSOs, an unbiased search of these absorbers requires a near-UV-selected QSO sample, as achieved by CUBS. CUBS LLSs exhibit multicomponent kinematic structure and a complex mix of multiphase gas, with associated metal transitions from multiple ionization states such as C II, C III, N III, Mg II, Si II, Si III, O II, O III, O VI, and Fe II absorption that span several hundred km s^–1 in line-of-sight velocity. Specifically, higher column density components (log N(H I)/cm^–2>= 16) in all four absorbers comprise dynamically cool gas with <T >=(2±1) ×10⁴K and modest non-thermal broadening of <b_nt >=5±3km s^–1. The high quality of the QSO absorption spectra allows us to infer the physical conditions of the gas, using a detailed ionization modelling that takes into account the resolved component structures of H I and metal transitions. The range of inferred gas densities indicates that these absorbers consist of spatially compact clouds with a median line-of-sight thickness of 160⁺¹⁴⁰_–50 pc. While obtaining robust metallicity constraints for the low density, highly ionized phase remains challenging due to the uncertain N(H I), we demonstrate that the cool-phase gas in LLSs has a median metallicity of [α/H]_1/2=-0.7^+0.1_–0.2, with a 16-84 percentile range of [α/H] = (-1.3, -0.1). Furthermore, the wide range of inferred elemental abundance ratios ([C/α], [N/α], and [Fe/α]) indicate a diversity of chemical enrichment histories. Combining the absorption data with deep galaxy survey data characterizing the galaxy environment of these absorbers, we discuss the physical connection between star-forming regions in galaxies and diffuse gas associated with optically thick absorption systems in the z < 1 circumgalactic medium.