X-Ray Detection of the Galaxy’s Missing Baryons in the Circumgalactic Medium of L* Galaxies

The number of baryons hosted in the disks of galaxies is lower than expected based on the mass of their dark matter halos and the fraction of baryon-to-total matter in the Universe, giving rise to the so-called galaxy missing-baryon problem. The presence of cool circumgalactic matter gravitationally bound to its galaxy's halo up to distances of at least 10 times the size of the galaxy's disk mitigates the problem but is far from being sufficient for its solution. It has instead been suggested that the galaxy's missing baryons may hide in a much hotter gaseous phase of the circumgalactic medium, possibly near the halo virial temperature and coexisting with the cool phase. Here we exploit the best available X-ray spectra of known cool circumgalactic absorbers of L* galaxies to report the first direct high statistical significance (best estimates ranging from 4.2σ to 5.6σ, depending on fitting methodology) detection of associated O VII absorption in the stacked XMM-Newton and Chandra spectra of three quasars. We show that these absorbers trace the hot medium in the X-ray halo of these systems at logT(in K) ≃ 5.8-6.3 (comprising the halo virial temperature T vir ≃ 106 K). We estimate masses of the X-ray halo within one virial radius within the interval ${M}_{\mathrm{hot}-\mathrm{CGM}}\simeq {(1\mbox{--}1.7)\times {10}^{11}(Z/0.3{Z}_{\odot })}^{-1}$ M ⊙. For these systems, this corresponds to galaxy missing-baryon fractions in the range ${\xi }_{b}={M}_{\mathrm{hot}-\mathrm{CGM}}/{M}_{\mathrm{missing}}\simeq {(0.7\mbox{--}1.2)(Z/0.3{Z}_{\odot })}^{-1}$ , thus potentially closing the galaxy baryon census in typical L* galaxies. Our measurements contribute significantly to the solution of the long-standing galaxy missing-baryon problem and to the understanding of the continuous cycle of baryons in-and-out of galaxies throughout the life of the Universe.