Massive warm/hot galaxy coronae - II. Isentropic model
Faerman, Y., Sternberg, A., McKee, C.
Abstract
We construct a new analytic phenomenological model for the extended circumgalactic material (CGM) of L<sup>∗</sup> 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<sub>gas</sub>≈5.5×10<sup>10</sup> M<sub>⊙</sub> inside r<sub>cgm</sub>≈280 kpc. The gas densities in the CGM are low (n<sub>H</sub>=10<sup>−5</sup>−3×10<sup>−4</sup> cm<sup>−3</sup>) 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<sub>OVI</sub>∼3×10<sup>14</sup> cm<sup>−2</sup> at large (≳100 kpc) impact parameters from the central galaxies, the ratio of the cooling to dynamical times, t<sub>cool</sub>/t<sub>dyn</sub>, has a model-independent upper limit of ≲4. In our model, t<sub>cool</sub>/t<sub>dyn</sub> 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.