A low density-wave spiral pattern speed, from the tracer separations
  (age gradient) across a spiral arm in the Milky Way
  We observe the density wave angular pattern speed OMEGA-p to be near 12 to 17
km / s / kpc, by the separation between a typical optical HII region (from the
spiral arm dust lane) and using a HII evolution time model to yield its
relative speed, and independently by the separation between a typical radio
maser (from the spiral arm dust lane) with a maser model.
