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We study the effect of baryons on the inner dark matter profile of the firstgalaxies using the First Billion Years simulation between z=16-6 before secularevolution sets in. Using a large statistical sample from two simulations of thesame volume and cosmological initial conditions, one with and one withoutbaryons, we are able to directly compare haloes with their baryon-freecounterparts, allowing a detailed study of the modifications to the dark matterdensity profile due to the presence of baryons during the first billion yearsof galaxy formation. For each of the ~ 5000 haloes in our sample we quantifythe impact of the baryons using eta, defined as the ratio of dark matter massenclosed in 100 pc in the baryonic run to its counterpart without baryons.During this epoch of rapid growth of galaxies, we find that many haloes ofthese first galaxies show an enhancement of dark matter in the halo centrecompared to the baryon-free simulation, while many others show a deficit. Wefind that the mean value of eta is close to unity, but there is a largedispersion, with a standard deviation of 0.677. The enhancement is cyclical intime and tracks the star formation cycle of the galaxy; as gas falls to thecentre and forms stars, the dark matter moves in as well. Supernova feedbackthen removes the gas, and the dark matter again responds to the changingpotential. We study three physical models relating the motion of baryons tothat of the dark matter: adiabatic contraction, dynamical friction, and rapidoutflows. Abridged, see text for full abstract

The First Billion Years project: dark matter haloes going from contraction to expansion and back again