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-of-sight velocity. Specifically, higher column density components (log N(HI)/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 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 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 $N\rm{(H\, {\small I})}$, 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.

Examining quasar absorption-line analysis methods: the tension between simulations and observational assumptions key to modelling clouds

“…In particular, single-phase ionisation modelling does not accurately capture the underlying distribution of metallicities and gas physical conditions (see Haislmaier et al 2021;Sameer et al 2021), even if it might capture the mean values (Marra et al 2021b;Sameer et al 2021). We expect that more complex, multi-phase modelling approaches (e.g., Zahedy et al 2019Zahedy et al , 2021Haislmaier et al 2021;Sameer et al 2021Sameer et al , 2022Nielsen et al 2022), will provide an improved characterisation of CGM gas. Specifically, the ionisation modelling method being developed by Sameer et al (2021Sameer et al ( , 2022 allows for multiple clouds in different phases of gas to contribute to a single VP component.…”

“…These insights present a significant challenge to analysis methods that employ Voigt profile (VP) modelling of the absorption profiles followed by chemical ionisation modelling constrained by the VP parameters, particularly when the analysis is based on the assumption that each VP component maps to a single discrete cloud having a single valued density, temperature, and metallicity. Some observational studies have explored chemical-ionisation modelling methods that treat the VP parameters of low-and highionisation absorption data (especially O absorption) separately, thereby attempting to capture the multi-phase characteristics of CGM gas (e.g., Muzahid et al 2015;Zahedy et al 2019Zahedy et al , 2021Haislmaier et al 2021). However, these methods force the ionisation modelling to conform to the VP fitted parameters, which may lead to systematic inaccuracies in the modelling outcomes (gas densities, temperatures, and metallicities) because the VP modelling is not informed by the gas physics but only by the kinematics of the absorption profiles.…”

Discovery of a damped Ly$α$ absorber originating in a spectacular interacting dwarf galaxy pair at $z = 0.026$

“…It is likely that the cross-section for ionized absorbers is larger still, but below the MUSE detection threshold; the 𝑧 = 0.364 Lyman limit system (LLS) detected in CUBS towards J0248−4048 arises from an analogous system of a relatively isolated dwarf galaxy pair at an impact parameter of several tens of kpc. The metallicity and chemical abundance pattern of the LLS suggests that it arises in tidal feature(s) composed of former ISM material (Zahedy et al 2021). The Magellanic Stream appears to be a local example of a multiphase feature whose formation was dominated by tidal forces from the Large Magellanic Cloud acting on its smaller companion on initial infall into the halo of the Milky Way (e.g., Besla et al 2012).…”

Chemical abundance scaling relations for multiple elements in z~2-3 star-forming galaxies

“…The CGM contains a significant fraction of all the met-als created in stars, and the distribution and physical state of this enriched gas can provide constraints on energetic feedback in galaxies (Tumlinson et al 2011;Peeples et al 2014;Oppenheimer et al 2016;Rudie et al 2019). Metal-bearing gaseous structures in the IGM and CGM at z ∼ 2 − 3 appear to be very small ( 100 pc; Rauch et al 2001;Rudie et al 2019), and there are significant variations in the metal content and abundance pattern of the CGM of single galaxies (Zahedy et al 2019(Zahedy et al , 2021Cooper et al 2021), suggesting that metals are often poorly mixed in the gas around galaxies. Future studies comparing the composition of the CGM with other components of the galaxy, including the ISM and stars, will offer critical insight into the baryonic flows that profoundly shape the evolution of galaxies.…”

“…These studies frequently use observations of the stellar continuum to measure, e.g., the iron, magnesium, and carbon content of the integrated stellar population; see Section 2 of Maiolino & Mannucci (2019) for a description of the methodology, but also Gallazzi et al (2005), which is the complementary study of stellar metallicity in the same galaxies studied by Tremonti et al (2004). More detailed abundance patterns are also central to studies of the circumgalactic medium (CGM) at all redshifts (Tumlinson et al 2017;Zahedy et al 2019Zahedy et al , 2021. More recently, very deep spectra of massive, quiescent galaxies at high redshift have enabled the measurement of multiple elemental abundances (Kriek et al 2016(Kriek et al , 2019Jafariyazani et al 2020).…”