The longstanding galaxy missing-baryon problem is solved by a large reservoir of hot gas permeating their halos

Abstract

The amount of baryons hosted in the disks of galaxies (stars and interstellar gas and dust) is lower than expected based on the mass of their dark-matter halos and the fraction of baryon-to-total matter in the universe[1], giving rise to the so called galaxy missing-baryon problem (e.g. [2]). The presence of cool circum-galactic matter gravitationally bound to its galaxy’s halo up to distances of at least ten times the size of the galaxy’s disk (e.g. [3-13]), mitigates the problem but is far from being sufficient for its solution. It has instead been suggested (e.g. [14]), that the galaxy missing baryons may hide in a much hotter gaseous phase of the circum-galactic medium, possibly at the halo virial temperature and co-existing with the cool phase. Here we exploit the best available X-ray spectroscopic data of known cool circum-galactic absorbers to show that the galaxy missing baryonic mass is indeed found in a hot medium permeating galaxy halos, at temperatures consistent with their virial temperatures. Main tracers of this hot medium are highly ionized light elements that only retain one or two electrons and that are only visible in the X-rays. Our measurements solve the long-standing galaxy missing baryon problem and thus significantly contribute to the understanding of the continuous cycle of baryons in- and-out of galaxies throughout the life of the universe.