Local Group Galaxies from an External Perspective

Abstract

I discuss Local Group galaxies from the perspective of external galaxies that define benchmark scaling relations. Making use of this information leads to a model for the Milky Way that includes bumps and wiggles due to spiral arms. This model reconciles the terminal velocities observed in the interstellar medium with the rotation curve derived from stars, correctly predicts the gradual decline of the outer rotation curve ($dV/dR = -1.7\;\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{kpc}^{-1}$), and extrapolates well out to 50 kpc. Rotationally supported Local Group galaxies are in excellent agreement with the baryonic Tully-Fisher relation. Pressure supported dwarfs that are the most likely to be in dynamical equilibrium also align with this relation. Local Group galaxies thus appear to be normal members of the low redshift galaxy population. There is, however, a serious tension between the dynamical masses of the Milky Way and M31 ($M_{200} \approx 1.4$ and $1.6 \times 10^{12}\;\mathrm{M}_{\odot}$, respectively) and those expected from the stellar mass-halo mass relation of abundance matching ($M_{200} \approx 3$ and $20 \times 10^{12}\;\mathrm{M}_{\odot}$, respectively).