The cool circumgalactic medium of low-redshift star-forming galaxies: I -- Empirical model and mean properties

Understand

We present an analytic model for the cool, $T \approx 10^4$ K, circumgalactic medium (CGM), describing the gas distribution, thermal and ionization state.

• Our model assumes (total) pressure equilibrium with the ambient warm/hot CGM, photoionization by the metagalactic radiation field, and allows for non-thermal pressure support, parametrized by the ratio of thermal pressures, $\eta = P_{\rm hot,th}/P_{\rm cool,th}$.
• We apply the model to the COS-Halos data set and find that a nominal model with $\eta = 3$, gas distribution out to $r \approx 0.6 R_{\rm vir}$, and $M_{\rm cool} = 3 \times 10^9~{\rm M_{\odot}}$, corresponding to a volume filling fraction of $f_{\rm V,cool} \approx 1\%$, reproduces the mean measured column densities of HI and low/intermediate metal ions (CII, CIII, SiII, SiIII, MgII).
• Variation of $\pm 0.5$ dex in the non-thermal pressure or gas mass encompasses $\sim 2/3$ of the scatter between objects.