DIFFUSE CORONAE IN COSMOLOGICAL SIMULATIONS OF MILKY WAY-SIZED GALAXIES

We investigate the properties of halo gas using three cosmological zoom-in simulations of realistic Milky Way-galaxy analogs with varying sub-grid physics. In all three cases, the mass of hot (T > 10(6) K) halo gas is similar to 1% of the host's virial mass. The X-ray luminosity of two of the runs is consistent with observations of the Milky Way, while the third simulation is X-ray bright and resembles more closely a very massive, star-forming spiral. Hot halos extend to 140 kpc from the galactic center and are surrounded by a bubble of warm-hot (T = 10(5)-10(6) K) gas that extends to the virial radius. Simulated halos agree well outside 20-30 kpc with the beta-model of Miller & Bregman based on O VII absorption and O VIII emission measurements. Warm-hot and hot gas contribute up to 80% of the total gas reservoir, and contain nearly the same amount of baryons as the stellar component. The mass of warm-hot and hot components falls into the range estimated for L* galaxies. With key observational constraints on the density of the Milky Way corona being satisfied, we show that concealing of the ubiquitous warm-hot baryons, along with the ejection of just 20%-30% of the diffuse gas out of the potential wells by supernova-driven outflows, can solve the missing baryon problem. The recovered baryon fraction within 3 virial radii is 90% of the universal value. With a characteristic density of similar to 10(-4) cm(-3) at 50-80 kpc, diffuse coronae meet the requirement for fast and complete ram-pressure stripping of the gas reservoirs in dwarf galaxy satellites.