The outer rotation curve of the Milky Way

A straightforward determination of the circular-speed curve v(c)(R) of the Milky Way suggests that near the Sun v(c) starts to rise approximately linearly with R. If this result were correct, the Galactic mass density would have to be independent of radius at R greater than or similar to R-0. We show that the apparent linear rise in v(c) arises naturally if the true circular-speed curve is about constant or gently falling at R-0 < R less than or similar to 2R(0), but most tracers that appear to be at R greater than or similar to 1.25R(0) are actually concentrated into a ring of radius R-1 similar or equal to 1.6R(0).