Massive Warm/Hot Galaxy Coronae. II. Isentropic Model

We construct a new analytic phenomenological model for the extended circumgalactic material (CGM) of L* galaxies. Our model reproduces the O VII/O VIII absorption observations of the Milky Way (MW) and the O VI measurements reported by the COS-Halos and eCGM surveys. The warm/hot gas is in hydrostatic equilibrium in an MW gravitational potential, and we adopt a barotropic equation of state, resulting in a temperature variation as a function of radius. A pressure component with an adiabatic index of g = 4 3is included to approximate the effects of a magnetic field and cosmic rays. We introduce a metallicity gradient motivated by the enrichment of the inner CGM by the Galaxy. We then present our fiducial model for the corona, tuned to reproduce the observed O VIO VIII column densities and with a total mass of MCGM >> 5.5 ' 1010 M..inside r >> 280 kpc CGM. The gas densities in the CGM are low (nH = 10- 3 ' 10-5- 4 cm-3), and its collisional ionization state is modified by the metagalactic radiation field. We show that for O VI-bearing warm/hot gas with typical observed column densities NO VI 3 ' 1014 cm-2.at large (.100 kpc) impact parameters from the central galaxies, the ratio of the cooling to dynamical times, tcool/tdyn, has a model-independent upper limit of.4. In our model, tcool/tdyn at large radii is 2 3 -. We present predictions for a wide range of future observations of the warm/hot CGM, from UV/X-ray absorption and emission spectroscopy to dispersion measure and Sunyaev-Zel'dovich cosmic microwave background measurements. We provide the model outputs in machine-readable data files for easy comparison and analysis.