Low-metallicity Absorbers Account for Half of the Dense Circumgalactic Gas at z ≲ 1

Abstract

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 Hubble Space Telescope 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 hosts a substantial mass of cool, dense, low-metallicity gas that may ultimately accrete onto the galaxies.