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Title: Where are the missing baryons of Milky Way-mass galaxies?
Plain: 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: For halos of $M_h \sim 1e12\,M_{\mathrm{sun}}$, 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 $1e4\,\mathrm{K}$, warm $1e5\,\mathrm{K}$, hot above $1e6\,\mathrm{K}$) inside the virial radius and the mass ejected beyond it.
Settled by: 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.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: partially-resolved
Status note: 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).
Posed since: (empty)