Where are the missing baryons of Milky Way-mass galaxies?
In plain words
A galaxy like ours should contain about one sixth of its total mass as ordinary matter, but stars and disk gas account for only a fraction of that. The rest may be in hot halo gas or may have been blown out of the halo.
Precise statement
For halos of $M_h \sim 10^{12}\,M_{\mathrm{sun}}$, stars and interstellar gas account for roughly 20 to 30 percent of $f_b M_h$ ($f_b \sim 0.16$, the cosmic baryon fraction). Determine the mass in each phase of halo gas (cool $10^{4}\,\mathrm{K}$, warm $10^{5}\,\mathrm{K}$, hot above $10^{6}\,\mathrm{K}$) inside the virial radius and the mass ejected beyond it.
What would settle it
Combined X-ray emission, Sunyaev-Zeldovich and fast-radio-burst dispersion measurements of the gas profile around a stacked sample of Milky Way-mass galaxies out to and beyond the virial radius.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
eROSITA stacking detects extended hot gas around Milky Way-mass galaxies, but its share of the budget remains debated (Zhang et al., A&A, 2026).
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