Abstract
We study the distribution of cold dark matter (CDM) in cosmological zoom-in
simulations from the Feedback in Realistic Environments (FIRE) project, for a
range of halo mass (10^9-10^12 Msun) and stellar mass (10^4-10^11 Msun). The
FIRE simulations incorporate explicit stellar feedback within the multi-phase
ISM. We find that stellar feedback, without any "fine-tuned" parameters, can
greatly alleviate 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 älpha" shows a strong mass dependence: profiles
are shallow at M_h ~ 10^10-10^11 Msun 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 älpha," and relatively late growth of cores.
Because the star formation efficiency is strongly halo mass dependent, a rapid
change in the central slope occurs at M_h ~10^10 Msun, as sufficient feedback
energy becomes available to perturb the DM. We show that 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. The same effects dramatically reduce
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, we study baryonic contraction in Milky Way-mass
halos. The net result of stellar feedback and baryonic contraction is to
produce DM profiles slightly shallower than the NFW profile, as required by the
normalization of the Tully-Fisher relation.
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