We analyze the physical conditions of the cool , photoionized ( T \sim 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 \sim L ^ { * } galaxies at z \sim 0.2 .
These data are well described by simple photoionization models , with the gas highly ionized ( n _ { HII } /n _ { H } \gtrsim 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 \pm 0.25 } ) ( R/R _ { vir } ) ^ { -0.8 \pm 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 \ion H1 column densities , we estimate a lower limit to the cool gas mass M _ { CGM } ^ { cool } > 6.5 \times 10 ^ { 10 } M _ { \odot } 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 _ { \odot } scale .