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)×104 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 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(HI) , 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.
Zahedy, F., Chen, H., Cooper, T., Boettcher, E., Johnson, S., Rudie, G., et al. (2021). The Cosmic Ultraviolet Baryon Survey (CUBS)-III. Physical properties and elemental abundances of Lyman-limit systems at z < 1. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 506(1), 877-902 [10.1093/mnras/stab1661].
The Cosmic Ultraviolet Baryon Survey (CUBS)-III. Physical properties and elemental abundances of Lyman-limit systems at z < 1
Cantalupo S.;
2021
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 ⟨T⟩=(2±1)×104 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 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(HI) , 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.
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