2016ApJ...830...87S

We develop a new method to constrain the physical conditions in the cool (∼10⁴ K) circumgalactic medium (CGM) from measurements of ionic column densities by assuming that the cool CGM spans a large range of gas densities and that small high-density clouds are hierarchically embedded in large low-density clouds. The new method combines the information available from different sightlines during the photoionization modeling, thus yielding tighter constraints on CGM properties compared to traditional methods that model each sightline individually. Applying this new technique to the COS Halos survey of low-redshift ∼L* galaxies, we find that we can reproduce all observed ion columns in all 44 galaxies in the sample, from the low ions to O VI, with a single universal density structure for the cool CGM. The gas densities span the range 50\lesssimρ/{bar}ρ_b\lesssim5×10⁵ ({bar}ρ_b is the cosmic mean), while the physical size of individual clouds scales as ∼ρ^–1, from ≃35 kpc for the low-density O VI clouds to ≃6 pc for the highest-density low-ion clouds. The deduced cloud sizes are too small for this density structure to be driven by self-gravity; thus, its physical origin is unclear. The implied cool CGM mass within the virial radius is (1.3 ± 0.4) x 10¹⁰ M_☉ (∼1% of the halo mass), distributed rather uniformly over the 4 decades in density. The mean cool gas density profile scales as R^–1.0±0.3, where R is the distance from the galaxy center. We construct a 3D model of the cool CGM based on our results, which we argue provides a benchmark for the CGM structure in hydrodynamic simulations. Our results can be tested by measuring the coherence scales of different ions.