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Cosmic Reionization On Computers: Baryonic Effects on Halo Concentrations During the Epoch of Reionization

Baryons both increase halo concentration through adiabatic contraction and expel mass through feedback processes. However, it is not well understood how the radiation fields prevalent during the epoch of reionization affect the evolution of concentration in dark matter halos. We investigate how baryonic physics during the epoch of reionization modify the structure of dark matter halos in the Cosmic Reionization On Computers (CROC) simulations. We use two different measures of halo concentration to quantify the effects. We compare concentrations of halos matched between full physics simulations and dark-matter-only simulations with identical initial conditions between $5 \leq z \leq 9$. Baryons in full physics simulations do pull matter towards the center, increasing the maximum circular velocity compared to dark-matter-only simulations. However, their overall effects are much less than if all the baryons were simply centrally concentrated indicating that heating processes efficiently counteract cooling effects. Finally, we show that the baryonic effects on halo concentrations at $z\approx5$ are relatively insensitive to environmental variations of reionization history. These results are pertinent to models of galaxy-halo connection during the epoch of reionization.