Great pull quote. For people new to the Tully-Fisher relation, the essence of it is that at galactic scale, gravity is not like v^2/r = GM/r^2. It's more like

Great pull quote. For people new to the Tully-Fisher relation, the essence of it is that at galactic scale, gravity is not like v^2/r = GM/r^2. It's more like v^2/r ~ sqrt(GM/r^2), so v^4 ~ M