Massive Warm/Hot Galaxy Coronae. II. Isentropic Model

Abstract

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 $\gamma =4/3$ 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 O VI-O VIII column densities and with a total mass of ${M}_{\mathrm{CGM}}\approx 5.5\times {10}^{10}$ ${M}_{\odot }$ inside ${r}_{\mathrm{CGM}}\approx 280\,\mathrm{kpc}$ . The gas densities in the CGM are low ( ${n}_{{\rm{H}}}={10}^{-5}\mbox{--}3\times {10}^{-4}$ 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 ${N}_{{\rm{O}}{\rm{VI}}}\sim 3\times {10}^{14}$ cm^-2 at large ( $\gtrsim 100$ kpc) impact parameters from the central galaxies, the ratio of the cooling to dynamical times, ${t}_{\mathrm{cool}}$ / ${t}_{\mathrm{dyn}}$ , has a model-independent upper limit of $\lesssim 4$ . In our model, ${t}_{\mathrm{cool}}$ / ${t}_{\mathrm{dyn}}$ at large radii is $\sim 2\mbox{--}3$ . 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.