The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium
  at z<1
  We assess the metal content of the cool (10^4 K) circumgalactic medium (CGM)
about galaxies at z<1 using an H I-selected sample of 28 Lyman limit systems
(LLS, defined here as absorbers with 16.2<log N(H I)<18.5) observed in
absorption against background QSOs by the Cosmic Origins Spectrograph on-board
the Hubble Space Telescope. The N(H I) selection avoids metallicity biases
inherent in many previous studies of the low-redshift CGM. We compare the
column densities of weakly ionized metal species (e.g., O II, Si II, Mg II) to
N(H I) in the strongest H I component of each absorber. We find that the
metallicity distribution of the LLS (and hence the cool CGM) is bimodal with
metal-poor and metal-rich branches peaking at [X/H]=-1.6 and -0.3 (or about
2.5% and 50% solar metallicities). The cool CGM probed by these LLS is
predominantly ionized. The metal-rich branch of the population likely traces
winds, recycled outflows, and tidally stripped gas; the metal-poor branch has
properties consistent with cold accretion streams thought to be a major source
of fresh gas for star forming galaxies. Both branches have a nearly equal
number of absorbers. Our results thus demonstrate there is a significant mass
of previously-undiscovered cold metal-poor gas and confirm the presence of
metal enriched gas in the CGM of z<1 galaxies.
