2014ApJ...792....8W

We analyze the physical conditions of the cool, photoionized (T ∼10⁴ 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_ H II_/nH_ ≳ 99%) by the extragalactic ultraviolet background. 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_CGM^cool > 6.5 x 10¹⁰ M_☉ for the volume within R < R_vir. Allowing for an additional warm-hot, O VI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10¹² M_☉scale.