Where are the missing baryons in clusters?
  Observations of clusters of galaxies suggest that they contain significantly
fewer baryons (gas plus stars) than the cosmic baryon fraction. This `missing
baryon' puzzle is especially surprising for the most massive clusters which are
expected to be representative of the cosmic matter content of the universe
(baryons and dark matter). Here we show that the baryons may not actually be
missing from clusters, but rather are extended to larger radii than typically
observed. The baryon deficiency is typically observed in the central regions of
clusters (~0.5 the virial radius). However, the observed gas-density profile is
significantly shallower than the mass-density profile, implying that the gas is
more extended than the mass and that the gas fraction increases with radius. We
use the observed density profiles of gas and mass in clusters to extrapolate
the measured baryon fraction as a function of radius and as a function of
cluster mass. We find that the baryon fraction reaches the cosmic value near
the virial radius for all groups and clusters above 5e13 solar masses. This
suggests that the baryons are not missing, they are simply located in cluster
outskirts. Heating processes (shock-heating of the intracluster gas, plus
supernovae and AGN feedback) that cause the gas to expand are likely
explanations for these results. Upcoming observations should be able to detect
these baryons.
