Kinematic Structure of the Milky Way Galaxy, Near the Spiral Arm Tangents

We compare the observed radial velocity of different arm tracers, taken near tangent to a spiral arm. A slight difference is predicted by density wave theory, given shock at entrance inner In many these arms, offset confirms prediction theory (with separation between maser and CO gas peak velocity, about 20 km/s) - when bigger than error estimates. The time reach next given.