The Impact of Baryonic Physics on the Structure of Dark Matter Halos:
  the View from the FIRE Cosmological Simulations
  We study the distribution of cold dark matter (CDM) in cosmological
simulations from the FIRE (Feedback In Realistic Environments) project, for
$M_{\ast}\sim10^{4-11}\,M_{\odot}$ galaxies in $M_{\rm
h}\sim10^{9-12}\,M_{\odot}$ halos. FIRE incorporates explicit stellar feedback
in the multi-phase ISM, with energetics from stellar population models. We find
that stellar feedback, without "fine-tuned" parameters, greatly alleviates
small-scale problems in CDM. Feedback causes bursts of star formation and
outflows, altering the DM distribution. As a result, the inner slope of the DM
halo profile ($\alpha$) shows a strong mass dependence: profiles are shallow at
$M_{\rm h}\sim10^{10}-10^{11}\,M_{\odot}$ and steepen at higher/lower masses.
The resulting core sizes and slopes are consistent with observations. This is
broadly consistent with previous work using simpler feedback schemes, but we
find steeper mass dependence of $\alpha$, and relatively late growth of cores.
Because the star formation efficiency $M_{\ast}/M_{\rm h}$ is strongly halo
mass dependent, a rapid change in $\alpha$ occurs around $M_{\rm h}\sim
10^{10}\,M_{\odot}$ ($M_{\ast}\sim10^{6}-10^{7}\,M_{\odot}$), as sufficient
feedback energy becomes available to perturb the DM. Large cores are not
established during the period of rapid growth of halos because of ongoing DM
mass accumulation. Instead, cores require several bursts of star formation
after the rapid buildup has completed. Stellar feedback dramatically reduces
circular velocities in the inner kpc of massive dwarfs; this could be
sufficient to explain the "Too Big To Fail" problem without invoking
non-standard DM. Finally, feedback and baryonic contraction in Milky Way-mass
halos produce DM profiles slightly shallower than the Navarro-Frenk-White
profile, consistent with the normalization of the observed Tully-Fisher
relation.
