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