Where Are the "Missing" Galactic Baryons?

Based on 19 high-resolution N -body/gasdynamical galaxy formation simulations in the ΛCDM cosmology, it is shown that for a galaxy like the Milky Way, in addition to the baryonic mass of the galaxy itself, about 70% extra baryonic mass should reside around the galaxy (inside of the virial radius), chiefly in the form of hot gas. Averaging over the entire field galaxy population, this "external" component amounts to 64%-85% of the baryonic mass of the population itself. These results are supported by the recent detection of very extended, soft X-ray emission from the halo of the quiescent, massive disk galaxy NGC 5746. Some of the hot gas may, by thermal instability, have condensed into mainly pressure-supported, warm clouds, similar to the Galactic high-velocity clouds (HVCs). Based on an ultra-high-resolution cosmological test simulation of a Milky Way-like galaxy (with a gas particle mass and gravity softening length of only 7.6 × 10 3 h -1 M ☉ and 83 h -1 pc, respectively), it is argued that the hot gas phase dominates over the warm gas phase, in the halo. Finally, an origin of HVCs as "leftovers" from filamentary, "cold" accretion events, mainly occurring early in the history of galaxies, is proposed.