ΛCDM‐based Models for the Milky Way and M31. I. Dynamical Models

We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard ΛCDM cosmology. The resulting models are confronted with the broad range of observational data available for the Milky Way and M31 galaxies. We find that there is a narrow range of parameters which can satisfy the observational constraints, but within this range, the models score remarkably well. Our favored models have virial masses 10 12 M ⊙ and 1.6 × 10 12 M ⊙ for the Galaxy and for M31 respectively, average spin parameters λ ≈ 0.03 − 0.05, and concentrations Cvir = 10−17, typical for halos of this mass in the standard ΛCDM cosmology. The models require neither dark matter modifications nor flat cores to fit the observational data. We explore two types of models, with and without the exchange of angular momentum between the dark matter and the baryons. The models without exchange give reasonable rotation curves, fulfill constraints in the solar neighborhood, and satisfy constraints at larger radii, but they may be problematic for fast rotating central bars. We explore models in which the baryons experience additional contraction due to loss of angular momentum to the surrounding dark matter (perhaps via a bar-like mode). These models produce similar global properties, but the