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 ∼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 (nH II /nH ≳ 99%) by the extragalactic ultraviolet background. Scaling by estimates for the virial radius, Rvir, 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 nH = (10–4.2 ± 0.25)(R/Rvir)–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 < Rvir. 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 1012M☼ scale. [ABSTRACT FROM AUTHOR]