The cos-halos survey: Physical conditions and baryonic mass in the low-redshift circumgalactic medium

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 LL* galaxies at z0.2. These data are well described by simple photoionization models, with the gas highly ionized (n _H II /n _H ≳ 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 _⊙ 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 10 ¹² M _⊙ scale.