Around The Way: Testing $\Lambda$CDM with Milky Way Stellar Stream Constraints

Recent analyses of the Pal 5 and GD-1 tidal streams suggest that the inner dark matter halo of the Milky Way is close to spherical, in tension with predictions from collisionless N-body simulations of cosmological structure formation. We use the Eris simulation to test whether the combination of dissipative physics and hierarchical structure formation can produce Milky Way-like galaxies whose dark matter halos match the tidal stream constraints from the GD-1 and Pal 5 clusters. We use a dynamical model of the simulated Eris galaxy to generate many realizations of the GD-1 and Pal 5 tidal streams, marginalize over observational uncertainties in the cluster galactocentric positions and velocities, and compare with the observational constraints. We find that the total density and potential of Eris contributed by baryons and dark matter satisfies constraints from the existing Milky Way stellar stream data, as the baryons both round and redistribute the dark matter during the dissipative formation of the galaxy, and provide a centrally-concentrated mass distribution that rounds the inner potential. The Eris dark matter halo or a spherical Navarro-Frenk-White dark matter work comparably well in modeling the stream data. In contrast, the equivalent dark matter-only ErisDark simulation produces a prolate halo that cannot reproduce the observed stream data. The on-going Gaia mission will provide decisive tests of the consistency between $\Lambda$CDM and Milky Way streams, and should distinguish between models like Eris and more spherical halos.