We present an analysis of the metallicity distribution of the dense circumgalactic medium (CGM) of galaxies at 0.1 z 1.1 as probed by partial Lyman limit systems (pLLSs, 16.1 < log N H I < 17.2) and LLSs (17.2 ≤ log N H I < 17.7 in our sample). The new H I-selected sample, drawn from our HST COS G140L snapshot survey of 61 QSOs, has 20 pLLSs and 10 LLSs. Combined with our previous survey, we have a total of 44 pLLSs and 11 LLSs. We find that the metallicity distribution of the pLLSs is bimodal at z 1, with a minimum at [X/H] = −1. The low-metallicity peak comprises (57 ± 8)% of the pLLSs and is centered at [X/H] −1.87 (1.3% solar metallicity), while the high-metallicity peak is centered at [X/H] −0.32 (48% solar metallicity). Although the sample of LLSs is still small, there is some evidence that the metallicity distributions of the LLSs and pLLSs are different, with a far lower fraction of very metal-poor ([X/H] < −1.4) LLSs than pLLSs. The fraction of LLSs with [X/H] < −1 is similar to that found in pLLSs (∼56%). However, higher H I column density absorbers (log N H I > 19.0) show a much lower fraction of metal-poor gas; therefore, the metallicity distribution of gas in and around galaxies depends sensitively on N H I at z 1. We interpret the high-metallicity ([X/H] ≥ −1) pLLSs and LLSs as arising in outflows, recycling winds, and tidally-stripped gas around galaxies. The low-metallicity pLLSs and LLSs imply that the CGM of z 1 galaxies is also host to a substantial mass of cool, dense, low-metallicity gas that may ultimately accrete onto the galaxies.
We present the results from our COS circumgalactic medium (CGM) compendium (CCC), a survey of the CGM at z 1 using H I-selected absorbers with 15 < log N H I < 19. We focus here on 82 partial Lyman limit systems (pLLSs, 16.2 ≤ log N H I < 17.2) and 29 LLSs (17.2 ≤ log N H I < 19). Using Bayesian techniques and Markov-chain Monte Carlo sampling of a grid of photoionization models, we derive the posterior probability distribution functions (PDFs) for the metallicity of each absorber in CCC. We show that the combined pLLS metallicity PDF at z 1 has two main peaks at [X/H] −1.7 and −0.4, with a strong dip at [X/H] −1. The metallicity PDF of the LLSs might be more complicated than an unimodal or bimodal distribution. The pLLSs and LLSs probe a similar range of metallicities −3 [X/H] +0.4, but the fraction of very metal-poor absorbers with [X/H] −1.4 is much larger for the pLLSs than the LLSs. In contrast, absorbers with log N H I ≥ 19 have mostly −1 [X/H] 0 at z 1. The metal-enriched gas probed by pLLSs and LLSs confirms that galaxies that have been enriching their CGM over billions of years. Surprisingly, despite this enrichment, there is also abundant metal-poor CGM gas (41-59% of the pLLSs have [X/H] −1.4), representing a reservoir of near-pristine gas around z 1 galaxies. We compare our empirical results to recent cosmological zoom simulations, finding some discrepancies, including an overabundance of metal-enriched CGM gas in simulations.
We characterize the metallicities and physical properties of cool, photoionized gas in a sample of 152 z ≲ 1 strong Lyα forest systems (SLFSs, absorbers with 15 < log N H i < 16.2). The sample is drawn from our Cosmic Origins Spectrograph (COS) circumgalactic medium compendium (CCC), an ultraviolet survey of H i-selected circumgalactic gas around z ≲ 1 galaxies that targets 261 absorbers with 15 < log N H i < 19. We show that the metallicity probability distribution function of the SLFSs at z ≲ 1 is unimodal, skewed to low metallicities with a mean and median of [X/H] = −1.47 and −1.18 dex. Very metal-poor gas with [X/H] < −1.4 represents about half of the population of absorbers with 15 < log N H i ≲ 18, while it is rare at higher N H i . Thus, there are important reservoirs of primitive (though not pristine) diffuse ionized gas around z ≲ 1 galaxies. The photoionized gas around z ≲ 1 galaxies is highly inhomogeneous based on the wide range of metallicities observed (−3 ≲ [X/H] ≲ +0.4) and the fact that there are large metallicity variations (factors of 2 to 25) for most of the closely spaced absorbers (Δv ≲ 300 km s−1) along the same sightlines. These absorbers show a complex evolution with redshift and H i column density, and we identify subtle cosmic evolution effects that affect the interpretation of metallicity distributions and comparison with other absorber samples. We discuss the physical conditions and cosmic baryon and metal budgets of the CCC absorbers. Finally, we compare the CCC results to recent cosmological zoom simulations and explore the origins of the 15 < log N H i < 19 absorbers within the Evolution and Assembly of GaLaxies and their Environments (EAGLE) high-resolution simulations.
We present a neutral hydrogen-selected absorption-line survey of gas with H i column densities at using the Cosmic Origins Spectrograph on the Hubble Space Telescope. Our main aim is to determine the metallicity distribution of these absorbers. Our sample consists of 224 absorbers selected on the basis of their H i absorption strength. Here we discuss the properties of our survey and the immediate empirical results. We find singly and doubly ionized metal species, and H i typically have similar velocity profiles, implying they probe gas in the same or similar environments. The ionic ratios (e.g., / , / ) indicate that the gas in these absorbers is largely ionized, and the ionization conditions are quite comparable across the sampled range. The Doppler parameters of the H i imply K on average, consistent with the gas being photoionized. The Mg ii column densities span >2 orders of magnitude at any given , indicating a wide range of metallicities (from solar to <1/100 solar). In the range of , there is a gap in the distribution corresponding to gas with ∼10% solar metallicity, consistent with the gap seen in the previously identified bimodal metallicity distribution in this column density regime. Less than 3% of the absorbers in our sample show no detectable metal absorption, implying that truly pristine gas at is uncommon. We find [Fe ii/Mg ii] , and since α-enhancement can affect this ratio, dust depletion is extremely mild.
We present a search for H I in the circumgalactic medium (CGM) of 21 massive ( log M ∼ 11.4), luminous red galaxies (LRGs) at z ∼ 0.5. Using UV spectroscopy of QSO sightlines projected within 500 kpc (∼R vir ) of these galaxies, we detect H I absorption in 11/21 sightlines, including two partial Lyman limit systems and two Lyman limit systems. The covering factor of log N (H I) ≥ 16.0 gas within the virial radius of these LRGs is f c (ρ ≤ R vir ) = 0.27 +0.11 −0.10 , while for optically-thick gas (log N (−0.07 . Combining this sample of massive galaxies with previous galaxy-selected CGM studies, we find no strong dependence of the H I covering factor on galaxy mass, although star-forming galaxies show marginally higher covering factors. There is no evidence for a critical mass above which dense, cold (T ∼ 10 4 K) gas is suppressed in the CGM of galaxies (spanning stellar masses 9.5 log M 11.8). The metallicity distribution in LRGs is indistinguishable from those found about lower-mass star-forming galaxies, and we find low-metallicity gas with [X/H] ≈ −1.8 (1.5% solar) and below about massive galaxies. About half the cases show super-solar [Fe II/Mg II] abundances as seen previously in cool gas near massive galaxies. While the high-metallicity cold gas seen in LRGs could plausibly result from condensation from a corona, the low-metallicity gas is inconsistent with this interpretation.
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