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

• Main Authors: Zahedy, Fakhri S (Author), Chen, Hsiao-Wen (Author), Cooper, Thomas M (Author), Boettcher, Erin (Author), Johnson, Sean D (Author), Rudie, Gwen C (Author), Chen, Mandy C (Author), Cantalupo, Sebastiano (Author), Cooksey, Kathy L (Author), Faucher-Giguère (Author), Greene, Jenny E (Author), Lopez, Sebastian (Author), Mulchaey, John S (Author), Penton, Steven V (Author), Petitjean, Patrick (Author), Putman, Mary E (Author), Rafelski, Marc (Author), Rauch, Michael (Author), Schaye, Joop (Author), Simcoe, Robert A (Author), Walth, Gregory L (Author)
• Format: Article
• Language: English
• Published: Oxford University Press (OUP), 2022-04-29T17:39:36Z.
• Subjects: Article
• Online Access: Get fulltext

 <jats:title>ABSTRACT</jats:title> <jats:p>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 $\langle T \rangle =(2\pm 1) \times 10^4\,$K and modest non-thermal broadening of $\langle b_\mathrm{nt} \rangle =5\pm 3\,$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 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 $N\mathrm{(H\, {\small I})}$, we demonstrate that the cool-phase gas in LLSs has a median metallicity of $\mathrm{[\alpha /H]_{1/2}}=-0.7^{+0.1}_{-0.2}$, 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.</jats:p>