Speaker
Description
The circumgalactic medium (CGM) of the Milky Way (MW) contains gas at a wide range of temperatures. The virial or warm-hot phase has a temperature of $\approx 2 \times 10^6$ K, while warm/cool gas ($\sim 10^4 - 10^6$ K) is also present. More recently, observations have revealed an even hotter component $(\sim 10^7$ K), referred to as super-virial' gas, detected through both X-ray emission and X-ray absorption in the spectra of background quasars. X-ray emission studies indicate that this hot gas is distributed across the entire sky and that there is a strong correlation between the emission measures (EM) of the virial and super-virial gas. In X-ray absorption, the existence of this hot gas $(\sim 10^7$ K) is identified through the presence of high-ionization metal lines such as OVIII, NeX, MgXII, and SiXIV in the spectra of three quasars. Along with high temperatures, the absorbing super-virial gas is found to have substantial column densities, and a non-solar composition with $\alpha$-element enhancement. Simple estimates suggest that the emitting and absorbing components of the super-virial gas have distinct physical origins. Based on these observational signatures, we propose that the emitting hot gas forms a puffed-up disk surrounding the MW, with a scale height of approximately 1 kpc and a radial extent of about 5 kpc. Disk-wide supernova-driven outflows from the MW disk could generate such a structure, that keeps dynamically evolving, leading to the high super-virial temperatures observed. This scenario is supported by numerical simulations using disk-wide outflows, with a radial profile of the star formation rate that is indicated in observations in our Galaxy. However, the puffed-up disk alone cannot account for the high ion column densities detected in absorption studies. The only other possibility of an extremely hot and $\alpha$-enriched gas is associated with supernova ejecta. We suggest that the absorbing column ofsuper-virial' gas ions originate in reverse shocks of core-collapse supernovae of runaway stars in extra-planar regions of the Milky Way, and which happen to lie along the three observed lines of sight. These reverse shocks can heat the supernova ejecta to extreme temperatures, consistent with that of the detected super-virial gas. Furthermore, core-collapse supernovae are naturally $\alpha$-enriched, aligning with the observed chemical composition. We show that for typical values of supernova projenitor mass and explosion energy, the observed column densities of high ionization species can be explained, in a time scale of a few hundred kyr. The prediction from our model is that the absorption signatures of the `super-virial' gas will have a rather small covering fraction in the sky, which will be tested in observations in the near future.