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.
