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

September 2021 • 2021MNRAS.506..877Z

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.

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 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.

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Greg Walth

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