2017ApJ...837..169P

We analyze new far-ultraviolet spectra of 13 quasars from the z∼0.2 COS-Halos survey that cover the H I Lyman limit of 14 circumgalactic medium (CGM) systems. These data yield precise estimates or more constraining limits than previous COS-Halos measurements on the H I column densities N_HI. We then apply a Monte-Carlo Markov chain approach on 32 systems from COS-Halos to estimate the metallicity of the cool (T∼10⁴ K) 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 H I surface density with impact parameter R_⊥ (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 10^–0.51 Z_☉ and a 95% interval ~1/50 Z_☉ to 3 Z_☉; the incidence of metal-poor ( 1/100 Z_☉) 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 N_HI (at 99.9 % confidence) and, therefore, also with increasing R_⊥; the high metallicity at large radii implies early enrichment; and (5) a non-parametric estimate of the cool CGM gas mass is M_CGM^cool=(9.2±4.3)×10¹⁰ M_☉, 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.