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 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  inside . The gas densities in the CGM are low ( 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 cm −2  at large ( kpc) impact parameters from the central galaxies, the ratio of the cooling to dynamical times, / , has a model-independent upper limit of . In our model, / at large radii is . 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.