Speaker
Description
Persistent small-scale challenges to the ΛCDM cosmological model have motivated the consideration of dark matter models with richer phenomenology. We consider a dark QCD scenario in which dark axions mediate a screened force between dark baryons within dark matter halos. Finite-density corrections to the dark QCD quark condensate introduce a density-dependent interaction term between dark axions and dark baryons, with a $\mathbb{Z}_2$ symmetry breaking, analogous to the symmetron mechanism. We use the FIRE-2 cosmological simulations, spanning dwarf to group halo mass scales, to test the feasibility of realistic dark matter halo profiles sourcing the dark axion. Through multi-objective optimization, we identify 3 example parameter sets that produce attractive forces of order $\sim 1-5$ times the strength of gravity, active over distances ranging from $\sim 50$ kpc to $\sim 1$ Mpc from the center of the halo, or $\sim 0.2 R_{\rm Vir}$ to $\sim 5 R_{\rm Vir}$ for a Milky Way-like halo. The force profiles generally follow the same structure: a screened center, a transition region where the force is active, and an outer decay to zero. Though our results only reflect the instant in which the axion is sourced, we tested this model against dynamical stability criteria including the free-fall time scale and Jeans length. These predict a spherical shell around the halo, aligning with the peak of the force profile, where circular orbits may be unstable and the halo is more vulnerable to collapse. The free-fall time is also lowered, suggesting that this DM model will result in large-scale rearrangement of the dark matter density.