N. Lehner, J. C. Howk, T. M. Tripp, J. Tumlinson, J. X. Prochaska, J. M. O'Meara, C. Thom, J. K. Werk, A. J. Fox, J. Ribaudo
We assess the metal content of the circumgalactic medium (CGM) about galaxies at z<1 using an HI-selected sample of 27 Lyman limit systems (LLS, defined here as absorbers with 16.2\lesssimlog N(HI)\lesssim 18.5 observed in absorption against background QSOs by the Cosmic Origins Spectrograph on-board the {Hubble Space Telescope. The N(HI) 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., OII, CII, MgII) to N(HI) in the strongest HI 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]simeq1.6 and -0.4 (2.5% and 40% 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 in the CGM of z<1 galaxies.
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http://arxiv.org/abs/1302.5424
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