Baryon Budget of the Hot Circumgalactic Medium of Massive Spiral
  Galaxies
  The baryon content around local galaxies is observed to be much less than is
needed in Big Bang nucleosynthesis. Simulations indicate that a significant
fraction of these "missing baryons" may be stored in a hot tenuous
circum-galactic medium (CGM) around massive galaxies extending to or even
beyond the virial radius of their dark matter halos. Previous observations in
X-ray and Sunyaev-Zel'dovich (SZ) signal claimed that $\sim(1-50)\%$ of the
expected baryons are stored in a hot CGM within the virial radius. The large
scatter is mainly caused by the very uncertain extrapolation of the hot gas
density profile based on the detection in a small radial range (typically
within 10\%-20\% of the virial radius). Here we report stacking X-ray
observations of six local isolated massive spiral galaxies from the CGM-MASS
sample. We find that the mean density profile can be characterized by a single
power law out to a galactocentric radius of $\approx 200\rm~kpc$ (or
$\approx130\rm~kpc$ above the 1~$\sigma$ background uncertainty), about half
the virial radius of the dark matter halo. We can now estimate that the hot CGM
within the virial radius accounts for $(8\pm4)\%$ of the baryonic mass expected
for the halos. Including the stars, the baryon fraction is $(27\pm16)\%$, or
$(39\pm20)\%$ by assuming a flattened density profile at $r\gtrsim130\rm~kpc$.
We conclude that the hot baryons within the virial radius of massive galaxy
halos are insufficient to explain the "missing baryons".
