Cuspy No More: How Outflows Affect the Central Dark Matter and Baryon
  Distribution in Lambda CDM Galaxies
  We examine the evolution of the inner dark matter (DM) and baryonic density
profile of a new sample of simulated field galaxies using fully cosmological,
Lambda CDM, high resolution SPH + N-Body simulations. These simulations include
explicit H2 and metal cooling, star formation (SF) and supernovae (SNe) driven
gas outflows. Starting at high redshift, rapid, repeated gas outflows following
bursty SF transfer energy to the DM component and significantly flatten the
originally `cuspy' central DM mass profile of galaxies with present day stellar
masses in the 10^4.5 -- 10^9.8 Msolar range. At z=0, the central slope of the
DM density profile of our galaxies (measured between 0.3 and 0.7 kpc from their
centre) is well fitted by rhoDM propto r^alpha with alpha \simeq -0.5 + 0.35
log_10(Mstar/10^8Msolar) where Mstar is the stellar mass of the galaxy and 4 <
log_10 Mstar < 9.4. These values imply DM profiles flatter than those obtained
in DM--only simulations and in close agreement with those inferred in galaxies
from the THINGS and LITTLE THINGS survey. Only in very small halos, where by
z=0 star formation has converted less than ~ 0.03% of the original baryon
abundance into stars, outflows do not flatten the original cuspy DM profile out
to radii resolved by our simulations. The mass (DM and baryonic) measured
within the inner 500 pc of each simulated galaxy remains nearly constant over
four orders of magnitudes in stellar mass for Mstar 10^9 Msolar. This finding
is consistent with estimates for faint Local Group dwarfs and field galaxies.
These results address one of the outstanding problems faced by the CDM model,
namely the strong discrepancy between the original predictions of cuspy DM
profiles and the shallower central DM distribution observed in galaxies.
