Evolution of the baryon fraction in the Local Group: accretion versus
  feedback at low and high z
  Using hydrodynamical zoom simulations in the standard LCDM cosmology, we
investigate the evolution of the distribution of baryons (gas and stars) in a
local group-type universe. First, with standard star formation and supernova
feedback prescriptions, we find that the mean baryonic fraction value estimated
at the virial radius of the two main central objects (i.e. the Milky Way and
Andromeda) is decreasing over time, and is 10-15% lower than the universal
value, 0.166, at z=0. This decrease is mainly due to the fact that the amount
of accretion of dissipative gas onto the halo, especially at low redshift, is
in general much lower than that of the dissipationless dark matter. Indeed, a
significant part of the baryons does not collapse onto the haloes and remains
in their outskirts, mainly in the form of warm-hot intergalactic medium (WHIM).
Moreover, during the formation of each object, some dark matter and baryons are
also expelled through merger events via tidal disruption. In contrast to
baryons, expelled dark matter can be more efficiently re-accreted onto the
halo, enhancing both the reduction of fb inside Rv, and the increase of the
mass of WHIM outside Rv. Varying the efficiency of supernovae feedback at low
redshift does not seem to significantly affect these trends. Alternatively,
when a significant fraction of the initial gas in the main objects is released
at high redshifts by more powerful sources of feedback, such as AGN from
intermediate mass black holes in lower mass galaxies, the baryonic fraction at
the virial radius can have a lower value (fb~0.12) at low redshift. Hence
physical mechanisms able to drive the gas out of the virial radius at high
redshifts will have a stronger impact on the deficit of baryons in the mass
budget of Milky Way type-galaxies at present times than those that expel the
gas in the longer, late phases of galaxy formation.
