The COS-Halos Survey: Metallicities in the Low-Redshift Circumgalactic
  Medium
  We analyze new far-ultraviolet spectra of 13 quasars from the z~0.2 COS-Halos
survey that cover the HI Lyman limit of 14 circumgalactic medium (CGM) systems.
These data yield precise estimates or more constraining limits than previous
COS-Halos measurements on the HI column densities NHI. We then apply a
Monte-Carlo Markov Chain approach on 32 systems from COS-Halos to estimate the
metallicity of the cool (T~10^4K) CGM gas that gives rise to low-ionization
state metal lines, under the assumption of photoionization equilibrium with the
extragalactic UV background. The principle results are: (1) the CGM of field L*
galaxies exhibits a declining HI surface density with impact parameter Rperp
(at >99.5%$ confidence), (2) the transmission of ionizing radiation through CGM
gas alone is 70+/-7%; (3) the metallicity distribution function of the cool CGM
is unimodal with a median of 1/3 Z_Sun and a 95% interval from ~1/50 Z_Sun to
over 3x solar. The incidence of metal poor (<1/100 Z_Sun) gas is low, implying
any such gas discovered along quasar sightlines is typically unrelated to L*
galaxies; (4) we find an unexpected increase in gas metallicity with declining
NHI (at >99.9% confidence) and, therefore, also with increasing Rperp. The high
metallicity at large radii implies early enrichment; (5) A non-parametric
estimate of the cool CGM gas mass is M_CGM_cool = 9.2 +/- 4.3 10^10 Msun, which
together with new mass estimates for the hot CGM may resolve the galactic
missing baryons problem. Future analyses of halo gas should focus on the
underlying astrophysics governing the CGM, rather than processes that simply
expel the medium from the halo.
