The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z<1. (arXiv:1302.5424v1 [astro-ph.CO])

The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z<1. (arXiv:1302.5424v1 [astro-ph.CO]):
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.