Should the Outer Milky Way be Axisymmetric?

Abstract

The shape of the outer Milky Way should be heavily influenced by the shape of our heavy halo. Simulations of the formation of collisionless halos indicate that they should be highly triaxial, with a tendency to pro-lateness. If our halo were as strongly triaxial as these simulations suggest, the outer disk would have ellipticity ϵdisk ≃ 0.06.Simulations of galaxy formation which include an appropriately small fraction of gaseous matter give rise to halos of much smaller ellipticity, and predict ϵdisk ≃ 0.02. These simulations are not as reliable as purely collisionless simulations, and we should treat their results with some caution. However, it is not incredible that a small fraction of gas could radically reduce the ellipticity of a halo by making the system's potential more centrally concentrated.The distribution of the apparent ellipticities of spiral galaxies indicates that their disks have slightly eliptical disks, ϵdisk ≃ 0.05. This may merely reflect ephemeral features such as spiral arms, however. The velocity fields of disks provide a more robust probe of the ellipticity ϵΦ of galaxy potentials. The narrowness of the Tully–Fisher relation places an interesting upper limit ϵΦ < 0.1. More detailed studies of individual velocity fields will be required if we are unambiguously to demonstrate ϵΦ > 0 in some cases.