The Cosmic Ultraviolet Baryon Survey (CUBS) – III. Physical properties and elemental abundances of Lyman-limit systems at z < 1. Issue 1 (22nd June 2021)

• Record Type: Journal Article
• Authors: Zahedy, Fakhri S; Chen, Hsiao-Wen; Cooper, Thomas M; Boettcher, Erin; Johnson, Sean D; Rudie, Gwen C; Chen, Mandy C; Cantalupo, Sebastiano; Cooksey, Kathy L; Faucher-Giguère, Claude-André; Greene, Jenny E; Lopez, Sebastian; Mulchaey, John S; Penton, Steven V; Petitjean, Patrick; Putman, Mary E; Rafelski, Marc; Rauch, Michael; Schaye, Joop; Simcoe, Robert A; Walth, Gregory L
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 506:Issue 1(2021)
• Page Start: 877
• Page End: 902
• Publication Date: 2021-06-22
• Subjects: surveys -- galaxies: haloes -- intergalactic medium -- quasars: absorption lines; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stab1661
• Languages: English
• ISSNs: 0035-8711

Abstract

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 &lt; 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 &lt; 1 circumgalactic medium.