We analyze the physical conditions of the cool, photoionized (T ∼ 10 4 K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of L ∼ L * galaxies at z ∼ 0.2. These data are well described by simple photoionization models, with the gas highly ionized (n HII /n H 99%) by the extragalactic ultraviolet background (EUVB). Scaling by estimates for the virial radius, R vir , we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n H = (10 −4.2±0.25 )(R/R vir ) −0.8±0.3 . Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the H I column densities, we estimate a lower limit to the cool gas mass M cool CGM > 6.5 × 10 10 M ⊙ for the volume within R < R vir . Allowing for an additional warm-hot, OVI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10 12 M ⊙ scale.

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Citation for published item:rohskD tF vier nd erkD tessi uF nd orsekD q¡ or nd rippD odd wF nd umlinsonD tson nd furhettD toseph xF nd poxD endrew tF nd pumglliD wihele nd vehnerD xiols nd eeplesD wolly F nd ejosD xiols @PHIUA 9he gyErlos survey X metlliities in the lowEredshift irumglti mediumF9D estrophysil journlFD VQU @PAF pF ITWF Further information on publisher's website: Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractWe analyze new far-ultraviolet spectra of 13quasars from thez 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 H I . We then apply a Monte-Carlo Markov chain approach on 32systems from COS-Halos to estimate the metallicity of the cool (T 10 4 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 parameterR (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 - to >  Z 3 ; the incidence of metal-poor (<  Z 1 100 ) 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 H I (at >99.9% confidence) and, therefore, also with increasingR ; the high metallicity at large radii implies early enrichment; and (5) a non-parametric estimate of the cool CGM gas mass is = ´ ( ) M M 9.2 4.3 10 CGM cool 10, 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.