The cosmic ultraviolet baryon survey (CUBS) – III. Physical properties and elemental abundances of Lyman-limit systems at z < 1

Abstract

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 FUV light from background QSOs, an unbiased search of these absorbers requires a NUV-selected QSO sample, as achieved by CUBS. CUBS LLSs exhibit multi-component 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-ofsight velocity. Specifically, higher column density components (logN(HI)/cm−2& 16) in all four absorbers comprise dynamically cool gas with 〈T 〉 = (2 ± 1) × 10 K and modest non-thermal broadening of 〈bnt〉 = 5 ± 3 km s−1. The high quality of the QSO absorption spectra allows us to infer the physical conditions of the gas, using a detailed ionization modeling 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 lineof-sight thickness of 160 −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 −0.2, 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.