  We study the central dark matter (DM) cusp evolution in cosmological galactic
halos. Models with and without baryons (baryons+DM, hereafter BDM model, and
pure DM, PDM model, respectively) are advanced from identical initial
conditions. The DM cusp properties are contrasted by a direct comparison of
pure DM and baryonic models. We find a divergent evolution between the PDM and
BDM models within the inner ~10 kpc region. The PDM model forms a R^{-1} cusp
as expected, while the DM in the BDM model forms a larger isothermal cusp
R^{-2} instead. The isothermal cusp is stable until z~1 when it gradually
levels off. This leveling proceeds from inside out and the final density slope
is shallower than -1 within the central 3 kpc (i.e., expected size of the
R^{-1} cusp), tending to a flat core within ~2 kpc. This effect cannot be
explained by a finite resolution of our code which produces only a 5%
difference between the gravitationally softened force and the exact Newtonian
force of point masses at 1 kpc from the center. Neither is it related to the
energy feedback from stellar evolution or angular momentum transfer from the
bar. Instead it can be associated with the action of DM+baryon subhalos heating
up the cusp region via dynamical friction and forcing the DM in the cusp to
flow out and to `cool' down. The process described here is not limited to low z
and can be efficient at intermediate and even high z.
