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Abstract

We construct a new analytic phenomenological model for the extended circumgalactic material (CGM) of L^∗ galaxies. Our model reproduces the OVII/OVIII absorption observations of the Milky Way (MW) and the OVI measurements reported by the COS-Halos and eCGM surveys. The warm/hot gas is in hydrostatic equilibrium in a 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 γ=4/3 is 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 OVI-OVIII column densities, and with a total mass of M_gas≈5.5×10¹⁰ M_⊙ inside r_cgm≈280 kpc. The gas densities in the CGM are low (n_H=10⁻⁵−3×10⁻⁴ cm⁻³) and its collisional ionization state is modified by the metagalactic radiation field (MGRF). We show that for OVI-bearing warm/hot gas with typical observed column densities N_OVI∼3×10¹⁴ cm⁻² at large (≳100 kpc) impact parameters from the central galaxies, the ratio of the cooling to dynamical times, t_cool/t_dyn, has a model-independent upper limit of ≲4. In our model, t_cool/t_dyn 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 (DM) and Sunyaev-Zeldovich CMB measurements. We provide the model outputs in machine-readable data files, for easy comparison and analysis.