The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z 1
We assess the metal content of the cool ~104?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.