Astrophys. J., 770, 138 (2013/June-3)
The bimodal metallicity distribution of the cool circumgalactic medium at z ≲ 1.
LEHNER N., HOWK J.C., TRIPP T.M., TUMLINSON J., PROCHASKA J.X., O'MEARA J.M., THOM C., WERK J.K., FOX A.J. and RIBAUDO J.
Abstract (from CDS):
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.
cosmology: observations - galaxies: abundances - galaxies: halos - galaxies: kinematics and dynamics
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<Available at CDS (J/ApJ/770/138): table1.dat table2.dat refs.dat>
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