Missing Halo Baryons and Galactic Outflows
  We present predictions for galactic halo baryon fractions from cosmological
hydrodynamic simulations with a well-constrained model for galactic outflows.
Without outflows, halos contain roughly the cosmic fraction of baryons,
slightly lowered at high masses owing to pressure support from hot gas. The
star formation efficiency is large and increases monotonically to low masses,
in disagreement with data. With outflows, the baryon fraction is increasingly
suppressed in halos to lower masses. A Milky Way-sized halo at z=0 has about
60% of the cosmic fraction of baryons, so "missing" halo baryons have largely
been evacuated, rather than existing in some hidden form. Large halos (>10^13
Mo) contain 85% of their cosmic share of baryons, which explains the mild
missing baryon problem seen in clusters. By comparing results at z=3 and z=0,
we show that most of the baryon removal occurs at early epochs in larger halos,
while smaller halos lose baryons more recently. Star formation efficiency is
maximized in halos of ~10^13 Mo, dropping significantly to lower masses, which
helps reconcile the sub-L* slope of the observed stellar and halo mass
functions. These trends are predominantly driven by differential wind
recycling, namely, that wind material takes longer to return to low-mass
galaxies than high-mass galaxies. The hot gas content of halos is mostly
unaffected by outflows, showing that outflows tend to blow holes and escape
rather than deposit their energy into halo gas.
