Dissecting Galaxy Formation: I. Comparison Between Pure Dark Matter and
  Baryonic Models
  We compare assembly of DM halos with and without baryons, within the context
of cosmological evolution in the LCDM WMAP3 Universe (baryons+DM, BDM model,
and pure DM, PDM model). In representative PDM and BDM models we find that
baryons contribute decisively to the evolution of the central region, leading
to an isothermal DM cusp, and to a flat DM density core -- the result of
heating by dynamical friction of the substructure during a quiescent evolution
epoch. This process ablates the cold gas from an embedded disk, cutting the
star formation rate by ~10, and heats up the spheroidal gas and stellar
components, triggering their expansion. The substructure is more resilient in
the presence of baryons. The disk which formed from inside-out as gas
dominated, is transformed into an intermediate Hubble type by z ~ 2 and to an
early type by z ~ 0.5, based on its gas contents and spheroidal-to-disk stellar
mass ratio. Only a relatively small ~20% fraction of DM particles in PDM and
BDM models are bound within the radius of maximal circular velocity in the halo
-- most of the DM particles perform larger radial excursions. We also find that
the fraction of baryons within the halo virial radius somewhat increases during
the major mergers and decreases during the minor mergers. The net effect
appears to be negligible. While the substructure is being tidally-disrupted,
mixing of its debris in the halo is not efficient and becomes even less so with
z. The streamers formed after z ~ 1 survive largely to the present time -- an
important implication for embedded disk evolution.
